The ubuntu repositories are split into four parts.
- main: free software supported by canonical
- restricted: non-free software supported by canonical (firmware, drivers etc)
- universe: free software not supported by canonical
- multiverse: non-free software not supported by canonical
Software not in main is not supported by canonical. The main practical impact of this for those of us who don't buy support contracts is that security updates are much less likely to appear in a timely manner and towards the end of the release lifecycle may not appear at all.
In general stuff gets into main because it's part of one of Ubuntus default installs. Gimp was dropped from the default install some time ago and I guess they just finally got around to removing it from main too.
Does that matter? In general I would say no, an image editor typically has relatively low exposure to security threats. Especially as the image formats themselves are typically handled by external libraries which (at least for the common formats) will probablly still be in main.
So to answer your "how to recover from it" question in the title I would suggest you just don't worry about it.