I'm skeptical of those GNP comparisons - Soviet GNP was always a tricky thing to measure and not really based on apples-to-apples figures like in most GNP country comparisons. At its largest, Soviet GNP was probably a third that of the US, and with a larger population (so a lower GNP per capita than the US). I explain more of that (with some further links to answers on the Soviet economy) here.

As for what happened with grain harvests, I'll try to run through this relatively concisely.

First, collectivization actually did not end Soviet grain exports - collectivization was undertaken in part to more efficiently gather grain for both consumption by industrial workers/urban inhabitants, and for sale abroad (to raise foreign currency to buy foreign capital goods). Grain exports actually increased substantially just after collectivization in the early 1930s, although horrifically this also coincided with famines killing maybe 7 million people across the USSR and malnourishing tens of millions more (and also coincided with a collapse in world grain prices, meaning that the USSR had to export more grain to earn the foreign currency it was looking to obtain).

This largely all fell apart with the Second World War - a significant number of the 26 million Soviet dead in that war were from war-related famines (both from lack of food in unoccupied territories, and forcible requisition/deliberate starvation under Axis occupation forces). The war damaged the agricultural sector severely, and even with inflated grain production targets in the later Stalin years, grain production even in the 1950s seriously lagged behind the 1940 output figures. This was in part because of wartime damages and the fact that postwar reconstruction was heavily focused on industry (especially the defense industry), over agriculture. Agriculture had massive postwar issues as well: newly-annexed territories in the West had to be collectivized, and wartime relaxations of central planning and collectivization were reversed. This was in an environment of widespread devastation and dislocation. Incentives were also small - collective farm workers earned a percentage of the farm's net income via "work days" contributed to the farm, but the farms had to sell to the state at set (low) prices and at set collection rates, so there was often little incentive to produce except through coercion (farmers tended to focus on their household plots, as they could consume whatever they produced and could sell the surplus at market rates, although this was taxed, regulated and monitored by the state). The situation was so bad that a postwar famine hit Moldova, Ukraine and Russia in 1946-1947, killing perhaps a million people.

Even after things began to slowly recover, major postwar changes meant that the USSR was not as heavily relying on grain sales for foreign currency (eventually hydrocarbons would fill this role starting in the late 1950s). Much of what was produced was consumed domestically (grain and potatoes were a heavy portion of the Soviet diet), or indirectly by livestock who in turn produced meat and dairy, which became themselves bigger goals under Khrushchev. Grain production became annual headline-level news all through the Soviet era(bringing in the grain harvest was the technical reason for the August 1991 coup, interestingly enough), but after 1963 consumption outstripped production, and net imports were needed by the USSR. What happened?

Agriculture in the Khrushchev years was still a massive sector of the Soviet economy. Even in the late 1950s over half of the Soviet population lived in rural areas, with almost a third of the Soviet population living on collective farms, although both these figures were steadily declining. Khrushchev, after Stalin's death, attempted a number of measures to jumpstart agriculture: compulsory requisitions from household plots were reduced, and prices for household plot goods and collective farm deliveries were raised (increasing agricultural income significantly). A massive plan was undertaken to increase the area under grain production by some 30 to 40 million hectares in the "Virgin Lands Campaign", which ploughed up lands in Siberia and Northern Kazakhstan starting in 1954, while investment was increased in agricultural production in southern Russia and Ukraine as well. What ultimately happened, however, was a focus on extensive versus intensive production. More area was put under grain production, but these new Virgin Lands were prone to drought and heavy erosion - weather changes could massively impact their output, as happened in 1963. Soviet agriculture was receiving more investment, but much of this increased output tended to get lost to wastage, or was being used for livestock. After a massive output increase in 1954-1958, agricultural output leveled off, and was prone to disastrous falls from droughts, such as in 1972, 1975 and 1981 (with foreign imports increasing those years, including in the 1980s from the United States).

In short, what happened was that Soviet citizens were consuming more agricultural output than in the 1930s, notably an increase in meat and dairy. Khrushchev and later Brezhnev were more concerned about consumer grumbles than Stalin ever was, and also were concerned with investing more in the agricultural sector (often facing opposition from "steel eater" industries like defense, that were competing for investments with agriculture). But the investments had increasingly small returns, often for the extensive-over-intensive reasons, and massive issues of micromanagement and wastage causing much agricultural output to be unused. Grain was no longer needed for exports, and the Soviet government was unwilling and unable to forcefully requisition agricultural supplies at the cost of causing starvation. Agricultural output grew, but never really took off, and the Soviet government often had to face bad weather conditions with foreign imports, rather than risk food shortages.