I'm wondering what your definition of a "full" pack is.
If you mean a 30-40L pack with overnight gear then it'll be a bit more of a grunt going up windy pass. You'll also find that it's difficult to get your head in a good riding position as your helmet will hit the pack for the steeper sections of the lick descent.
If you mean a reasonable size pack with all your tools, spare long sleeve, rain jacket, food, at least 2L of water and a few first aid items. Then that's the minimum for what you should be using anyway for most rides in the Chilcotins or any remote back country area.
Gun creek trail is a fun trail with some good views on the upper sections (don't forget to stop and look behind you for the best views). It's only about 3 hours from the lake back to the road and then another 30-45 back to Tyax. So pretty short.
If you want the epic views then up over Windy to lick is pretty awesome. Until last year we said I'd never ride/hike up in Windy from spruce (we've done it the other direction). It wash't actually as bad as we'd thought. I was actually able to ride more than I would have thought.