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.