I just checked on the .GOP registry and both the registry and registrar are operational.
We reported a few hours ago that the .GOP registry and registrar were not operational
To register a .GOP domain name visit Join.GOP
Domain names are priced at $20.16
Of course there are reserved and restricted domains but there is a form on the site if you are interested on buying one of those.
It appears that Join.Gop is the only place that you can register a .GOP domain name, MindsandMachines.com does not seem to be accepting registrations yet.
To learn more about the .GOP extension you can visit their official site at DotGop.co