Automatically scan each chat message for words like gift, steam, card, cash, etc ....
If a word is detected, whoever sent it has their profile secretly flagged. Then, somebody at Grindr can block accordingly or whatever they need to do.
If you want to pretend you care about privacy, you can have a pop-up come up saying we think you received chat message from a scammer, may we have access to the chat you had with them.