Yeah, GIMP doesn't have many options when it comes to interpolation. And Photoshop is really expensive ;-;
if GIMP had/has a feature that imports>"video frames to layers" like Photoshop, I wouldn't even need Photoshop. I do all my things in GIMP beside that one thing.
another thing GIMP needs is multi-layer selection. the lack of it is possibly the worst thing about it.
edit: like so

i only used photoshop to import a video file to make the layers, deleted irrelevant layers, saved, opened in GIMP>made gif through process(cropping, scaling, timescale, etc)
I prefer GIMP over Photoshop when it comes to animated gifs. When scaling and cropping normal flat images, GIMP is faster.