The first big difference is about popularity.
Gratuit has been more popular than gratis since centuries and the trend is highly in favor of the former.
There is also the colloquial/slang gratos which usage is slightly raising (and the spoken French usage is likely higher than what books analysis exhibits):
Another difference is the type of usage. I'm not sure of the truthfulness of Google Ngram classification algorithms but it detects extremely low uses of gratis as an adjective, up to one thousand less than adverb use:
The trend is also to in favor of gratuitement against gratis (gratuitement is used 12 times more nowadays):
This, combined with fact the gratis ends with an s so wouldn't change between singular and plural anyway makes *gratisse unlikely to appear.
Same for gratos and an hypothetical *gratosse...