Find out what codes are being set. You can have them read at most auto parts stores.
It's my understanding that ABS/Stabilitrack lights will disable the trailer brakes, so I suspect there's no real problem with them.
Also, try another dealer. Is there a GMC dealer nearby? They'll work on it.
1988 Coleman Sequoia - popup (1987-2009)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10