I would take 70 to GJ go south on hwy 50 then take 141 to gateway and down to Nucla and then across to 90 to la sal jnct.
There are a bunch of little roads from gateway to castleton or moab but I've never been on them. I'm guessing they are dirt roads...
I've only ever driven the 128 and 191 routes.
However, in checking MapSource (the routing software from Garmin), it appears that there's a route from just outside of Gateway, directly into downtown Moab.
Here are the major junctions (according to MapSource):
Gateway (on the 141, south of GJ)
Go south on 141 (about one mile)
At John Brown Rd, turn Right (West) on John Brown Road
At Road 5/10 (6 miles), make a hard left to stay on John Brown
At Road 1/10 (1.4 miles), make a hard left onto Gateway
At Castleton Rd (2.5 miles), turn/stay left onto Castleton
At Gateway (8.9 miles), turn/stay left onto Gateway
At Lasal Mountain Loop (3.2 miles), turn left onto Lasal Mountain Loop
At Sand Flats Rd (4.8 miles), turn Right onto Sand Flats Rd
At Mill Creek (9.6 miles), turn/stay right onto Mill Creek
At Road 400, turn right onto 400
At Center, turn left onto Center
in 4 blocks you're at Hwy 191 in downtown Moab
In checking the route in Google Earth, it appears to be mostly a "truck trail", and would probably be sketchy going through the La Sals if there's a late snow.