Thanks for doing those tests.
It must be something specific to the GIF and MP4 types if it's working with BMP, but nothing stands out as wrong in your GIF file type diagnostic from earlier. I've tried recreating the same values here without any issues as well, as far as possible. (I don't have all the same apps installed, so one of them could be related, but it shouldn't matter.)
Windows 10 has a command line method of assigning file types which has worked in the past when the Open With menu didn't. It might be worth a try to use that method, described in this post:
(Instead of .png use .gif, and instead of pngfile use giffile -- note the double-f in the last one there.)
If you want a quick workaround, another thing you can do in Opus is:
- Settings > File Types.
- Double-click the GIF type in the System File Types part of the list.
- Select the Events tab and double-click the Left double-click event.
- Click the Browse button above the main editing area, and locate the exe of the viewer you want to use, and click OK in the browse dialog.
- Add
{filepath}after the exe path, and click OK.
That will change what Opus does when the type is double-clicked, and also what the Enter key does. If a Left double-click event is defined then Opus won't consult the registry etc. at all, as it overrides everything. It also won't affect what other programs do, so you can have different viewers in Opus and everywhere else if desired.