Oh, wow, are you going? It is fantastic! I have been there in January and in March. Neither time was there any difficulty driving due to snow on the roads. The mountains are too sheer to have roads. You have to go through a tunnel to get into the valley, and you can't drive out the other side--the only access is rail. You actually put your car on a train and go that way.
The Gasteinertal (Gastein Valley) is a rather broad, flat expanse with mountains on all four sides. There are several small towns in the valley. Bad Gastein was (I felt) the most interesting. It is at the far end of the valley and kind of "crawls up" the mountain, so it's very hilly. It looks completely unlike the other Alpine mountain towns I have been to. The buildings are very large and ornate, and look more like they belong in Vienna than the Alps. It was a spa town built for the rich and famous, including the Emperor, and it looks the part.
I really doubt there would be snow in May. It's not at a really high altitude.
Laurie, let me know if you have questions about the area. We stayed at the Mondi Bellevue, which is lovely.
