iPhone photos app 'syncing' lots and not showing photos in other apps since updating to iOS 8.3 and probably enabling some new sort of iCloud system, all my photos in the photos app showed a white circular progress indicator in the bottom right the first time I looked at the picture, which seemed related to network activity.

No idea why this was happening but ended up noticing that a gif viewing app I was developing was suddenly showing many fewer gifs than before. Upon further inspection i found that those gifs I hadn't looked at in the photos app, and hence hadn't done their white progress indicator thing, were missing. Once I'd let them do their white progress indicator thing they showed up again.

So I went through almost all my photos and gifs in the photos app to let them all get 'synced' (or whatever they're doing) so all my gifs would show up and then I noticed some that I'd previously 'synced' were missing again and required another 'sync' to show up again... What?? 

Has anyone else experienced this? Does anyone know what's going on?
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.