How to detect when gigfile has finished loading? How to detect when rackspace is ready?

IMO you should definitely be making use of this. Myself and others normally use OSC as an easy mechanism to communicate between the Gig Script and Local Global rackspace scripts. You could also likely inject a midi message into the Local GP Port, and use the midi callbacks to pick this up in a global rackspace.

As you are getting more adventurous, you may also want to look into extensions, such as this one that @Frank1119 created: