Gumo was my first real backpacking experience. It is a unique place with incredible contrasts. The escarpment (Bush Mountain) is worth the effort and according to the rangers something few people see.
Once on top (Pine Top, Permian Reef, Tejas Trail, McKittrick Canyon campground) you hit pine, oak, and maple forests. Hard to believe that you are in Texas, or that forest can be found in those mountains. Especially since all you see as you approach Gumo is rock, dirt, cactus, and rattlers...
Day hikes: the peak, devil's canyon & devil's staircase
The wind can blow you off the mountain. I am not kidding. The frequency of high winds drops significantly in May/June.
Too hot in July/August.
Nice in late October/November (and the maples are beautiful).
Must pack water. No fires!!