Why doesn't Google Drive show up in the list of apps when sharing a photo? Open the Photos app
Select an image
Pull up the menu (bottom left icon)
Available apps are Message, Mail, Twitter, etc.


Google Drive does not show up in that list (which seems to be called "Activities").

I have signed in to the Google Drive app and even restarted my phone.
I just had the same issue here in August 2018 on iOS 11.4.  I am using the Apple-native Photos app and installed Google Drive two days ago, when I successfully was able to share from the Photos app onto Google Drive.  Now, two days later, it isn't working.  I tried the Save to Files option but Google Drive doesn't show up there either.

Turns out, it appears that I have selected too many photos at one time to upload.  When I select ten or fewer photos to share, it shows the Google Drive icon in the Activities list as an option.  But when I select more than ten photos, the Google Drive icon disappears from the Activities list.  Must be some kind of internal limit placed on it or perhaps a total upload size limit.  So if I want to upload more than ten photos, I will just do more than one upload.