What you're experiencing is one major issue raised by iPad users. Yes, you are far better using the Gmail App than the native Gmail on iPad. Even with the native, if you switch to desktop view, you'll have the same issue.
My issue has been, in the native version, can't attach anything without issues. Using the Gmail App, I'm able to attach, but only pictures. It doesn't allow me to open other subfiles to attached pics in other files or PDF, Word,Excell,etc which I create using QuickOffice Pro.
I've tried this on various Android Tablets, and not a single issue with them. You can attach anything. So, it's definately an Apple device thing.
I'll even go further. If using Facebook on an iPad 3, you can post, but cannot delete a post you make whether in desktop view, native FB on iPad or the FB App itself. Yet, on my Droid Razr phone, I can by simply swyping sideways, a black X appears and I delete the post.
It's these quirks, that is leading me to consider selling my iPad 3, and just stick with my 13" MacBook Pro.