Welcome to OFF, and to our area.
GP is a great place. I've fished it OFF, and on, since the mid 1960's. But, mostly for trout and sometimes kokes. Kokes are a landlocked sockeye salmon. They are just about the best fish I've ever had SMOKED! Plus, you can keep 25 kokes per day. There are also large Chinokanee too (landlocked Chinook). A fellow OFFer got one last month, that was in the 10 lb. range!
I don't recall seeing any Yaks out there. It's a big lake. But, I do know that peeps fish from Yak's, canoe's, etc. It's do-able.
WORD: Almost every single day, a strong wind whips the waves up at 9:30 or 10:00 a.m. So, keep that in mind when you're planning to go.