Take I-70 to the Bakerville exit (#221). Leave the highway and drive south over to the dirt parking area near the start of Forest Road 189. This is the winter trailhead and, even if the upper road is open, low-clearance passenger cars should park here. It's almost 3 miles to the summer trailhead. Reach a junction after one mile - stay straight and follow the sign for the Grays Peak trailhead. Continue another 2 miles to the trailhead at 11,280'. There are restrooms and a few dispersed camping spots near the parking area.
First, cross the bridge that spans the stream in Stevens Gulch and follow the well-defined Grays trail up the hill into Stevens Gulch. Photo #1 is the view of Grays from near 11,700'. In the distance, and east of the Grays summit, you can see Lost Rat Couloir - Photo #2. Continue on the Grays trail for nearly 1.5 miles until you are near the trail sign near 12,100'. Just before the sign, locate a small road that leaves the left side of the main trail - Photo #3.
Follow the small road down a bit and then south towards the center of the basin. Photo #4 is another close-up view of Lost Rat. After heading south for a bit, continue southwest on the road as you approach the southwest end of the basin - Photo #5. Stay on or near the road as it climbs southwest up onto the talus below Lost Rat - Photo #6. From this location, you have an excellent view of the couloir - Photo #7 and Photo #8. Hike over to the base of Lost Rat (Photo #9). Crampons, axe, helmet, and avalanche gear are recommended.
Start climbing. The terrain quickly steepens above 12,800', as you approach the confines of the couloir - Photo #11. Taken before the couloir narrows, Photo #12 shows the area and might give you an idea of the steepness. Above 13,100', reach a fork in the couloir - Photo #13. A steeper, narrow couloir leads up to the right, but stay left and continue climbing up the main couloir. Photo #14 looks down from 13,200'. The next 300' of the couloir is straightforward and the slope angle reaches 45 degrees - Photo #15, Photo #16, Photo #17, Photo #18 and Photo #19. Near 13,500', the route turns slightly left as you pass through the narrowest section of the couloir - Photo #20. Climb the last 100' (Photo #21 and Photo #22) to reach the top of Lost Rat - Photo #23. Photo #24 and Photo #25 look down on the couloir and Photo #26 is a broad look at the top.
Turn right (west) to see the remaining 700' up Grays' East Ridge - Photo #27. Follow the ridge (Photo #28 and Photo #29) to the summit - Photo #30.
The slope angle in Lost Rat reaches 45 degrees and much of the rest is close to 40 degrees. While it isn't as long as the Dead Dog Couloir on Torreys, the steepness is similar, with the exception of a short section near the top of Dead Dog, which may exceed 45 degrees.