Just what it says on the tin folks:

ProtoGun can now be played straight from the browser, as embedded HTML5 (with optional full-screen)
Downloads for all supported platforms are still available (Windows / Mac / Linux) but for those who just want to have a play straight away, or are perhaps cautions about downloading and running unsigned/self-signed executables, you're now set like a jelly!

Only shame is the screenshots have vanished from the main page, a consequence of how itch.io handles HTML5 game pages, but for the convenience of playing it a supported browser, I suppose it'll be a-ok ;)
I've since tweaked the formatting and embedded the screenshots as a combined image. Not perfect, but it did sort out the problem!