As near as I can tell, Chrome isn't showing in the share sheet for other apps either, which means you can't open links to Chrome from anywhere except inside Chrome itself.
There was an update to Chrome yesterday. My guess is it broke whatever method is used to register file-type compatibility wiht iOS. Gmail probalby uses the same hooks as the share sheet to detect Chrome and use it by default.
I tried deleting and redownloading Chrome to see if it would help, but no change.
I doubt very much this was intentional, and expect there will be another update to fix the issue rather soonish. Well, we can hope.
Once upon a time there used to be a setting in Gmail that let you choose Safari or Chrome as the default. It's either been removed, or it only shows up when it detects the Chrome share sheet (which isn't happening).