Have you checked the Greybanded forum (72)?
>>What are the best spots for herping (esp for Trans Pecos rats and gray banded kings) in west tx where it is legal to collect a couple breeding specimens?
There are no best spots anymore, what with all TX roads now designated as "wildlife sanctuaries" and illegal to hunt (including cuts on the right-of-way). 
And what are the best techniques for these species?
Spend every summer (all of it) timing your trips to the new moon and hope to either get lucky and find an alterna your first trip, or make the average 1 find per 5 years! I've yet to find one, even when road cruising was legal.
If you can find private land with paved roads, and permission to hunt, that would be the easiest way to find them. Subocs are a little easier to find (based on my own experience of subocs-2, alterna-0 ). Joining HCU could be a good way to gain access to land.
>>Also, is it legal to collect from Black Gap Wildlife Management?
No--you can hunt other animals there, but not herps.
I know many people do, but is it legal?
The roads used to be fair game, but alas, no more.
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Chris McMartin
www.mcmartinville.com
I'm Not a Herpetologist, but I Play One on the Internet