"opera_gx_splash" itself is a nuisance that I'm sure never made a single user happy. Opera has long become a browser to be avoided, except for one or other feature, just like any other thing made only for invasive marketing purposes.
I'm glad GX still uses the not-so-broken traditional UI, now that "Opera One" UI is a broken and unusable thing since its birth.
You can use a webp format to get a gif background. if you cant download it as a webp you can turn a gif into a webp using https://ezgif.com/gif-to-webp its easy to use.