I've only ridden GJ a couple times (Ribbon - Lunch area) and it was pretty fun. A bit techy for me in places, but nothing I couldn't avoid (as my travel partners did 10 ft drops and I found the trails around such stuff.)
Breck is where I live and there are infinite choices up here. I personally prefer the West Ridge (CO trail) from Middle Fork up the CO and then back down to the Dredge TH (I think it was previously mentioned to ride up from the Dredge.) You can also hit tons of fun stuff from the Dredge TH. If you want more climbing, find French Gulch (road) on the map, follow that road up to Little French (tough climb), and then you can fire down a flume trail to Lincoln Meadow. Many trails back in this area as well. Peaks trail is always fun, although I prefer riding it from Frisco to Breck...don't know why really, I just like that route even though it's more uphill. If you want a real breathtaking ride (as it has great views and spends a lot of time above treeline) you can ride up Peak 9 to Wheeler trail, then stay right on Miner's Creek Trail which will dump you out in Frisco (decent amount of hike-a-bike on some of the upper sections, and it's probably longer than 3 hours, unless you're REALLY fast...even then probably 4.5+) If you start up Baker's Tank and then stay right rather than descending left (the right is how the Firecracker 50 goes), you can get to some fun stuff (check out the map for the Firecracker 50 and you'll see Pinball Alley as well as Nightmare on Baldy...and on the later part of the loop Barney Ford and Moonstone, which you may have ridden in the past.) Best quick ride, at least one of my favorite quickies, is the Flumes trails. They start near the 7-11 on the south end of town (actually just above the recycling center.) Can loop the Upper/Middle/Lower in about 30 minutes...repeat as desired.
Hope some of this is helpful.