How do I add the Gmail app to my sharing options (i.e. from photos)?
The coders at Gmail need to add it, it's not a user option, I'm afraid.

If Google add it, then it has to go past Apple's reviewers, to check it's as safe as they think it is.  Sandboxing is pretty tight under iOS.  

The feature you want is not impossible, it just needs to be implemented at source, not added by the user.  

The end result is no-one needs an anti-virus scanner on an iDevice ;) 

Try reporting it on the Gmail feature requests page