Gnb is pretty easy as far as mtn airports go. Lots of busy airspace around cos, I'd avoid it all but that's just me.
As you said, depart 27. It's a big valley and you have a lot of room to maneuver and climb. Corona pass is right there to the se, depending on where you're coming from you could head up over bjc and then hit Rollins/Corona, it's an easy to make at 12,5 pass but that might run you afoul of my suggestion to avoid the military and training happening around cos and puts you along the rocks where it will be bumpy. Due west of gnb is twin peaks and some pretty high terrain but there's a pass there too, the name escapes me.
Be there around or before noon.
It's an easy shot up that valley to a lot of other airports too.
27 is right pattern, hill in the way of left. There's a little shack with keys to a loaner car. It's a ragged out Ford suv that used to be a police car.
Co dot has a free map with preferred routes and the Colorado pilots association website has good suggestions. I've flown turbos too long and lost my edge on my flying.
In a 231 I'd just file 180 and cancel round about Denver and start down to cross Corona at 13ish and maneuver in a big arc around the west side to join a 45 for 27 right downwind. But I'm lazy.
I go to lava Java cafe for breakfast, it's good but not mind blowing. Gnb is a 20 minute flight or 3 hour drive from me so it's fun to take people there.