So from what I recall from the books of Beevor's, Stahel, and Glantz, the fate of the average German soldier captured on the eastern front varied on the time period, course of the war, military campaign, etc.

The early prisoners captured during 1941 were rare and relatively valuable. They were needed for interrogation and as such Stalin made several General Orders testifying to the need for their humane treatment. However, sudden catastrophes facing the Red Army on successive encirclements, the relatively high-morale amongst the winning German forces, and the relatively few successes of the Red Army in the summer of 1941 resulted in few prisoners taken. Frantic retreat and the NKVD's own cruelty led to several massacres of many captured Germans. It is to be noted that there were several anecdotes of German POWs tortured to death too, especially in the desperate encirclement battles of Kiev and Smolensk. However, such behavior could change between campaigns. Multiple Russian accounts during the triumphant winter battles around Moscow note that frostbitten Germans were generally spared and sent to the rear (Jones, The Retreat). Enough thousands survived to participate in a staged Stalinist procession through Moscow.

This early volatile treatment changed as the war carried on. A combination of Soviet propaganda, reports from rescued POWs, and fear led to the Red Army growing more and more enraged towards their national enemy, and from 1941-42 this was reflected by ad hoc and hopelessly disorganized treatment of prisoners, often culminating in summary executions. From 1941 until Stalingrad, the rations (esp. water) necessary to keep prisoners alive did not seem to be present in the gulag system, either out of callous hate, indifference, or actual logistical limitations. Many Germans were quite literally worked to death, having not enough food to survive. Stalingrad was an anomaly in some ways: already half-starved and given little consideration by the Soviet transit system, very few frozen Germans survived the first few months of imprisonment.

From 1943-45, surrender became a safer option. However cruel the Stalinist system was, it also published multiple orders citing the need for prisoners for both economic and tactical reasons (labor and easier victories). There were large-scale massacres, most notably involving an overrun German field hospital in Ukraine, that were never punished. There were multiple occasions where retreating Germans were massacred in the act of surrendering. At Korsun, General Konev sent cavalry and T-34s to smash apart retreating columns, and such actions were repeated during many breakout attempts. It is to be noted that Soviet tactical doctrine generally sent pamphlets and radio broadcasts to encourage encircled Germans to surrender. These large-scale massacres were of soldiers surrendering 'too late' for Soviet liking.

We have a notion nowadays that Soviet troops rarely accepted German surrenders, and that this is the reason the war in the east was harsher. This is not true. The Soviet Union processed upwards of 4.5 million Axis prisoners, albeit not only from Germany, but from all the Axis countries. Summary executions did occur, but they were more often the fate for German soldiers trying to surrender alone or in small groups, especially if they did so amidst a battle, where Soviet casualties would obviously enrage survivors. When units surrendered en bloc or when deserters surrendered on quiet sectors, they were generally far safer.

The SS and rear detachments generally faced death if caught in small numbers. The partisan war was a fierce horror, and throughout the entire war, neither side generally took prisoners unless specifically requested by the high command. The SS moreover, had a horrible reputation in the occupied East. Survivors were often shot on sight, to the general indifference of the Soviet high command. Reasons for this treatment include the fact that the SS served in operations as camp guards, extermination 'anti-partisan' units, vanguard attack troops, or rearguard scorched earth operations. None of these functions would generally mellow feelings amongst the Soviet soldier.

Nonetheless, the average German would survive surrender. What followed however, might very well be a slower death. It is notable that gulag prison conditions were (somehow!) a marked improvement over the 'death marches' that initially took prisoners away from the front. The death toll on forced marches and overburdened trains was often obscenely high (and ironically normally higher than German train routes in the concentration camp system. The Stalinist gulag however was tried to actively profit from prisoner labor, unlike the German concentration camps for Jews, gypsies, communists or Slavs. This reflected Generalplan Ost's goal to eliminate the intelligentsia and potential for resistance amongst the Slavs.

Consignment to the gulag was neither predictable nor inevitable. At the very tail end of the war, many prisoners were only kept incarcerated for several months, in similarity with the standard treatment of German POWs found by the Anglo-Americans. However, most prisoners who survived summary execution at the end of battle faced several years of required labor. Some historians note the high losses amongst these prison laborers (Eby). However, most of those imprisoned from 1943-45 survived to return home. It is to be noted that a prisoner's fate, like many others in the Soviet Union, were often amazingly varied and ludicrously random. Nor were soldiers the only prisoners. Even civilians might be conscripted by the Stalinist quotas for labor. Some prisoners would fare relatively well with high rations and decent treatment, especially in the south. Some trainloads were even left half-forgotten in small hamlets. Others would be less fortunate, overworked in harsh Siberian conditions.

Suffice it to say, it is accepted that roughly 25-35% of 'fascist' prisoners in Soviet custody died. The survivors were normally released back to their home countries within a few years, often broken in body or mind from the horrors of total war.