Are all Appimages using that, if not what percentage of the ones you know?
Usually if the appimage has a github release with a zsync you have that verification.
And are tools like Gearlever enforcing or using that signature check?
I don’t use gearlever, as far as I know gearlever doesn’t even let you sandbox the appimage like AM does. I don’t think any of those forces signature verification besides AppImageUpdateTool and that’s because that’s part of the zsync update process.
Anything portable.
aisap uses bwrap it is mentioned in both links I gave you.
appman used to have firejail sandbox but it was dropped in favor of aisap because of that.