The trailhead for McCullough Gulch uses the same directions as for Quandary Peak, initially. If coming from Breckenridge on SH 9, drive south from the center of town approximately 8 miles where you turn right (west) onto "Blue Lakes Road #850.
If coming from Hoosier Pass on SH 9, drive north from the summit of the pass just over 2 miles to the left turn (west) onto Blue Lakes Road #850.
NOTE: As of July 30, 2021, parking for either Quandary Peak or McCullough Gulch has been regulated. A parking permit must be obtained in advance at a significant cost, or you must ride a shuttle in from the airport north of Breckenridge to the trailhead. Check out this link for more info: https://www.parkquandary.com/ The shuttle will drop visitors off at the intersection described below. If you do not want to "pay-to-play" or utilize the shuttle, then an unregulated access can be found by using the Spruce Creek-Mohawk Lakes TH described on this site.
Once on CR 850, drive past the large, day-use parking lot and make a right turn (north) onto CR 851. This is the same road that takes you to the well-used east ridge approach for Quandary. If arriving here on a summer weekend, drive past the multitude of cars parked along the road left by 14er baggers, and continue north, to NW for 1.7 mile to an intersection. The trailhead coordinates provided are for this intersection. If you can find a place to park, do so nearby to begin your hike. The road to this point is typically drivable by passenger vehicles. If no parking is readily available, take the left fork and you may be able to drive another .5 mile to a locked gate and a place where you can park at 11,090 ft. at these coordinates: N 39° 24' 03.99" W 106° 04' 45.23". Alternately, you can also take the right fork and drive down to the creek and park.
Overnight parking at any of the parking areas for Quandary is prohibited. Some at-large camping may be available in the area of the trailheads identified, especially down by McCullough Creek. But if arriving on a summer weekend, most any decent spot will likely be taken and these may now be prohibited as well. There are no National Forest campgrounds in the immediate vicinity. For campgrounds located near Dillon Res., try this site for more information: http://www.townofdillon.com/visit/camping-info.