Interesting… I’m about 99% sure the problem is that Gliffy appears to respond to HEAD requests with a 404, which is waaaaaay wrong. Like, “bro do u even RFC?!?”-grade wrong. You can work around that by using a proper markdown “include image” link, like this:
Which renders correctly:
I have my suspicions that, even if you could get Gliffy to fix their bone-headed handling of HEAD requests, though, you probably won’t get the result you want. Discourse usually downloads externally-linked images and replaces them with inline versions, to avoid the problem a lot of forums have of old posts being graveyards of broken image links (and also to avoid inadvertently putting load on other people’s servers). There might be a setting somewhere to turn off downloads for a whole site, or some magical incantation to disable it for a given image link, but I don’t know what that would be, if it exists.
Myself, I ditched Gliffy a while ago and am a happy user of draw.io. There’s ways to embed whole diagrams, and I have a dream, one day, that I’ll get around to building a plugin to provide a little “Edit with draw.io” link next to such diagrams, at which point I will immediately expire in a paroxym of ecstasy.