I've been playing with RD Web this week, and was getting the following error when attempting to Connect to a remote PC - "remote desktop gateway server is temporarily unavailable" on my Server 2012 R2 test rig.
I'm able to use Remote Desktop Connection if I manually specify my gateway server settings.
After much Googling it occurred to me that it could be because I'm running my RDWeb server on an unusual external port. I've already had to configure the HTTP and UDP Transport settings to match my bespoke port (RD Gateway Manager properties), so I drilled down into the Application Settings in IIS Manager and added :PORT# to the end of my FQDN in the DefaultTSGateway field.
I've ended up with something along the lines of: rdw.domain.com:1234
This subtle change (after a refresh of the REWeb portal in the browser) seems to have done the trick.
