What John said: play it on the open horn. It will speak more easily and with better timbre than on the Gb valve.
I've been teaching myself bass trombone for about a year and currently am what I call "not awful" at it. I concur with John in his approach. There are two schools of thought on the valves: (1) Use them only when you must, and (2) Since you have them, use them. I started out pretty much with (2), but have now switched to a kind of intermediate approach which is essentially what John describes. Also, since I have independent valves, there are times when the Gb in the staff in 1st position (well, flat first, or what Aharoni calls the 2nd valve first position) is just real handy.
I find that I am playing more notes without the valves (particularly sixth and seventh position notes), both because it is better slide technique in a lot of places and because it speaks more easily and sounds better. And I am using the F valve much more than the Gb valve, and use the double valves only for a very few (and infrequently encountered) notes.
I harbor a somewhat odd belief that an independent double-valve bass trombone would be better to set up with Gb as the first (and more frequently used) valve, and F as the second. I've thought about doing that to my horn, but it requires some degree of effort. And an additional downside is that it requires the development of habits that are incompatible with all "normal" trombones with F/Gb or F/D attachments. Still, it keeps attracting me.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)