Before you battle Venam in the gym you are told to be aware of field effects. However, there seem to be none, as usually there is a flavor text showing you one is active. You can actually see what happens in Shofu's Let's Play (18:22):
I noticed in the previous versions it said "This field is corrupted" and it was pretty obvious the effects were active as switching pokémon damaged them (which also does not happen in the Let's Play above). To fix it for myself, I digged in the scripts of the game and in "PokeBattle_ActualScene" I didn't find any mention of the "Poison" backdrop in the metadata of the gym's maps. There are, however, "Poison" bases and background in the Battlebacks folder, but no "Corrosive" ones, which according to the script seem to be correct ones. I don't know if any of this is supposed to happen or not. Lastly, changing in the script every mention of "Corrosive" (including the quotes) to "Poison" (including the quotes) fixes this "bug".
tl;dr As it stands, the first gym seems to have no field effects whatsoever. Check video above.
