Why doesn't Google Drive show up in the list of apps when sharing a photo?
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.