This is an important question to answer because periodically on Reddit, TIL posts bring up the early Soviet regime decriminaltion homosexuality without either contextualizing or examining it. What follows is largely a discussion of the USSR, as I am not certain as to what the exact policies were like in the Warsaw Pact states.

Homosexuality was never really tolerated within the USSR, but that Soviet state expressed that intolerance in various ways. What's usually referred to as the decriminialization of homosexuality was the USSR was really the repeal of sodomy laws. The tsarist state saw sodomy as a religious sin and since the tsar was the nominal head of the Orthodox church, enforcing this law became part of the state's religious policy. The Bolsheviks inherited this legacy of these religious-based laws and they rejected them.

The repeal of sodomy laws does not mean that the Soviet state saw homosexuality in neutral terms. Although few Bolsheviks had devoted time to the sexual question, those that did saw that homosexuality was a deviant behavior that the modern Soviet state would solve through psychology rather than the religious means that typified the tsarist regime. Soviet psychologists saw homosexuality like other mental health issues: a disorder caused by larger society. They associated homosexuality with bourgeois decadence and aesthete society that would have no place within the new Soviet Union (with a sizable minority ascribing to the theory that homosexuality was a glandular problem). Sexual reformers arguing that sex was an individual's right for pleasure, like Alexandra Kollantai, were a distinct minority. According to Healey, Lenin denounced liberal sexual theories that emphasized sex was for the freedom of the individual as “modern fashion” and “ignorant bungling,” and interest in these issues was “a hobby of the intellectuals and of the sections nearest to them.”

Thus when the Stalinist state reintroduced anti-sodomy laws along with a five year prison sentence in the 1930s it was not so much a departure from the 1920s as a change in tone. Some of the rationale behind this was the conservative social turn of Stalin in which sodomy laws went along with restrictions on abortion and other family laws. However, the Stalin era shares a stark line of continuity in many of its attitudes towards homosexuals as a evidence of bourgeois decadence. The continuation of such practices was even more out of place since Stalinism declared that it had created a new society purged of these elements. With the rise of fascism, there were those that began to argue that among fascism's many crimes was sexual deviance. One example of this thinking came from the author Maxim Gorky, who asserted "destroy the homosexuals- fascism will disappear!"

Attitudes towards homosexuality softened slightly in the medical community post-Stalin, the criminal penalties remained on the books. The notion that homosexuality was deviant behavior became entrenched within much of Soviet government and bureaucracy. Evidence of homosexuality treatments in various mental hospitals were a black mark that was hard to erase. Convicted homosexuals were denied certain employment opportunities and had restricted movements.

As to what being gay was like in the USSR, is a far more difficult answer. Certainly homosexuals had to operate under these highly restrictive laws, but a gay subculture did exist. In some areas like the Gulags, Red Army or massive new cities like Magnitogorsk same-sex segregation allowed for increased opportunities for same sex contact. It's also notable that the Soviet authorities seldom bothered themselves with female homosexuality. Soviet medical discourse tended to emphasize female sexuality as a component of biological reproduction; female sex with other women (or even men!) for pleasure did not really fit into these preconceptions.

Edit: a few typos and clarifications

Sources

Fitzpatrick, Sheila. Everyday Stalinism: Ordinary Life in Extraordinary Times : Soviet Russia in the 1930s. New York: Oxford University Press, 1999.

Healey, Dan. Homosexual Desire in Revolutionary Russia: The Regulation of Sexual and Gender Dissent. Chicago: University of Chicago Press, 2001.

_. Bolshevik Medicine and Russia’s Sexual Revolution

I can only speak about Poland. Homosexuality was never illegal there (at least when Polish state existed, during partitions in XIX century laws of Prussia, Austria and Russia were relevant, respectively). In 1932 an equal age of consent has been set for heterosexual and homosexual (15 yo). The exception has been gay prostitution which has been criminalised until 1969.

That being said, Poland has not been a very accepting country when it comes to homosexuality, mainly due to heavy influence by Catholic Church. The communist authorities were pretty conservative as well when it comes to sexual matters. Homosexualism was a matter which basically did not exist as a public topic. It has been though used by secret police for blackmailing gay people into cooperation with them. The most known is "Operation Hiacynth" which took place in years 1985-1987 and included systematic infiltration of gay community to gather blackmail material and, as a result, gain information on anticommunist oposition. The official purpose of this operation was containment of AIDS epidemic and acting against gay prostitution. National database of all Polish homosexuals and people who were in touch with them was being created as part of the operation and it resulted in registering around 11,000 persons.

While Operation Hyacinth has been the most infamous and wide-spread action taken by secret police against gay community, blackmail of homosexuals was being used by them thorough the whole communist period. Another well known instance is forcing Foucault to leave Poland in 1959 (he was at the time a lecturer at Warsaw University).

Yugoslavia wasn't in the Eastern Bloc but I feel it's related to your question.

Homosexuality was criminalized after WWII and the main issue then was whether the disorder was bourgeois and decadent or just human. It was decriminalized in the 70's when Slovenian and Croatian medical professionals and intellectuals no longer considered it a mental disorder but remained banned in the rest of Yugoslavia except Montenegro, Vojvodina and the aforementioned Croatia and Slovenia. Most US states, Austria, Norway and Finland decriminalized it at about the same time so that was on par with international developments.

More liberal parts of the federation developed gay scenes, magazines, radio shows and associations that dealt with gay rights, although Belgrade had a strong gay scene despite the ban in Serbia. There were underground night clubs and known meeting places; gays were harassed as much as other western imports such as punk so there were a few moral panics by state controlled media but that wasn't the official party stance.

Rumor has it that some important Yugoslav partisans and later politicians were gay, but they weren't prosecuted although an average punishment for homosexual activity in the 50's was between one and five years in jail.

An interesting thing is that there are quite a few popular songs about homosexuality from that time and there wasn't much fuss about it.

For your listening pleasure:

Idoli - Retko te viđam s devojkama - I Rarely See You With Girls

Xenia - Moja prijateljica - My Girlfriend (read more like "my female friend")

Videosex - Ana - Includes "Ana why do I you like crazy?"

KUD Idijoti - Preživjeti - Hommage to a band member who died of AIDS