I have kept corys on gravel. They did fine for me, but if your gravel is sharp ended it can harm there barbels. If you do get them, just watch the barbels ( feelers on mouth)
Honestly they are happier on sand but can still live on gravel.
I kept my corys in a 55 g for awhile now, and they were just fine on the gravel, but went. OCD on the sand, sifting through it and spitting out the sand through the gills.