I just checked my own stash and saw the default for CR2 being an older Topaz app, so it's not specifically a Gigapixel issue.
I'll ask the devs about this and see what's up.
PS: I took 2 seconds and changed it to Photoshop 2022 ;-)
Update: I have asked the devs and will report back.
I'm curious if you ever dragged a RAW file directly into Gigapixel? Perhaps this is what resets the file association (?). Let's see what others say.