iPhone photos app 'syncing' lots and not showing photos in other apps
What settings do you have under Settings app > iCloud > Photos? I'm assuming you have iCloud Photo Library turned on and Optimize iPhone Storage checked. See this. 

Regarding the thumbnails not showing up in your GIF app, it's likely due to the way the app reads your photo library and probably hasn't been updated to account for Optimized Storage with iCloud Photo Library.

In an app that reads the library correctly, the thumbnails of all photos should show up, and for a photo whose full size isn't cached on your device, it will download at the time you tap on it.