How do I add the Gmail app to my sharing options (i.e. from photos)?

I've installed the Gmail app on my iPhone 5s (running iOS 8.x). However, when I'm done taking a photograph (or directly from within the Photos app), if I choose to share I'm presented with a list of options (Mail, Messages, Facebook, etc.) and the Gmail app isn't one of them.

If I go a step further and click the More link to see the list of all apps on the phone for which sharing can be enabled, the Gmail app isn't even in that list (while things like Slack.com and other apps I use are).

I use the stock Mail app for my work (Exchange) account, and I could if I wanted share a photo just fine using this, however I prefer the Gmail app's categories of mail.

Do you know a set of steps that can get the Gmail app to appear in this list of sharing options?

Any pointers would be greatly appreciated, please advise if you need more details or clarification.