I texted a GIF from my Pixel and it was really blocky, even at the highest setting. It doesn't look bad during playback or when uploaded to Photos though. Is this an MMS issue or something?
I just used it on my S7 and shared it on whatsapp: looks very high quality to me. You can try changing the gif quality from settings, although the default worked fine for me.
Either way, the app is neat and I think this is just an artifact in the MMS stage rather than the files.