AFAIK gifv is actually a webm or mp4 video without sound and not a "real" new format, so I think Opus can be persuaded to recognize it by setting its extension in Opus' Movie Viewer Plugin (found in Preferences / Viewer / Plugins):
As for webp, it gained limited support in Opus some time ago (you can even use Opus to directly convert it into other image files), but Opus has no animated support for it.