Hells Gate is an optional obstacle on Hells Revenge - 4+ But I have cooked through that thing with 12 rigs behind me in 90 minutes...

Gemini Bridges - 2.5 to 3 - is its own trail, although you exit/enter onto it for many trails:
Gold Bar Rim - 3.5+ (part of Golden Spike - 4+)
Rusty Nail - 4+ (which can venture into GS)
Metal Masher - 4
and several others (Bull Canyon - 3)

3D is 3 trails in one all ~3-3.5 - Hidden Canyon, Hidden Canyon Overlook, Tusher Tunnel

Seven Mile Rim - 3.5, Wipe Out Hill - 4

Lockhart Basin 3.5

Behind the Rocks - 4+ but there is a bypass for virtually everything making it Tiptoe Behind the Rocks - 3.5

That was from memory, so verify here:
Trails
check here:
Trails part deux