In GURPS system is a player able to call upon the being that blessed him using channeling or is the spirit that enters your body randomly determined? The description says:
Once you have entered your trance, any spirit in the immediate vicinity can enter your body and use it to speak or write messages.
But it doesn't speak of whether you can call upon something to come to you when you go into a trance.
GURPS is definitely a Rule Zero game: the GM's word is final, so anyone else's opinion is only advisory. A meta-rule in GURPS is "you get what you buy" and trying to draw implications from detailed examination of rules isn't something that the rules-writing is designed to support.
I'm pretty sure that the combination of Blessed, which is a trait about being attuned to a major power, and Channelling, which is about letting spirits of any power level communicate through you, don't have such a synergy. This being GURPS, you can of course create one.
The simple way to do that is to take the being that has Blessed you as a Patron, with the Highly Accessible modifier. With that, you can contact it anywhere, any time, if you make your Patron roll. Having it communicate with you through Channelling then becomes a sensible implementation detail. This is a sensible way to deal with beings that are gods, or operate in a similar way. Having Medium would be an alternative to Channelling, but talking to them via Medium might be considered presumptuous by deities.
Another way to do it is to take Medium as well as Blessed and Channelling. This is appropriate for major spirits that are not gods: having Medium means you can call to them, but it's up to them if they answer.
