Frankly, wouldn't you assume that the gasterbeiters of ATL would be Eastern European instead of Turkish?
I think there necessarily would be work-related migration from Eastern Europe to Western Europe, as the latter would most likely be on average a more affluent area. But then I also believe that when moving across borders for work is allowed, better-off nations will always attract people from less successful economies. So, at least parts of Eastern Europe would ITTL get guest workers from outside Europe as well. This all of course has many moving parts - how does decolonization pan out ITTL, how does the Middle East develop, or northern Africa. What is the trajectory of the lands of the (possibly former) Russian Empire. I think that in some Eastern European countries we could see brain drain to the West, being then offset by getting guest workers from countries outside Europe. And then we should not even entirely discount moving from the West to the East, in terms of our definitions. Greeks or Portuguese workers moving to Czechoslovakian factories to build cars would not be impossible ITTL. Like I pointed out earlier, some nations in Eastern Europe ITTL would be more affluent than others, and some even more affluent than a part of what we see as Western nations.
So, while I agree that moving from the East to the West inside Europe would be common, more common than IOTL, it would not make up the whole picture. IOTL, countries have inside 20-40 years moved from sending guest workers to receiving them - like Finland, where great numbers of people moved to Sweden for work in the 70s and which since the 90s has received similarly major numbers of Estonian guest workers. ITTL we would see similar developments with nations changing places, as it were, depending on the success of their policies, the general European and world trends and contingency. It also might be that we could have success stories we can't predict or unexpected losers. Western Europe would not be unaffected, and we can't count on all "Western" nations doing as well as IOTL in the face of competition from a large number of Eastern European economies, unfettered by the chains of real Socialism and the Iron Curtain like IOTL. In this I am mainly looking at the "PIIGS" as potential comparative losers.
Refer to this map:
Without World War II, Poland would stay as an agrarian authoritarian dictatorship on the outskirts of Europe, so it's best to compare it to another agrarian authoritarian dictatorship on the outskirts of Europe, which had basically the same level of GDP per capita:
Spain.
While I agree with what you are saying here*, I am not entirely trusting of the numbers in those maps, namely the 1938 one. I for example doubt Finland in 1938 had passed France and the Netherlands in GDP per capita, or that the gap to Estonia, say, was
that big. I think generally IOTL Finland approaching the bigger Western economies so closely happened only in the 70s and 80s.
So, personally I'll reserve some room for scepticism in terms of these maps.
*To an extent. I don't exactly believe in fully predetermined economic trajectories for nations, so there would have been room for Poland to grow into a more affluent economy than Italy, I believe, with the right decisions and a measure of luck. Not as much room as for Czechoslovakia, but it would exist.