May I ask why it HAS to be an external gate? Generally, the external gate turbos are fairly large ones.
You've said that you want a reasonable responsive one - tha'ts a bit of an issue, especially with the rb20 isn't it.
The best information you can give, is how much power you want to achieve, then we can tell you which turbo will make that power, and how laggy it will be on the 20
A GCG rb25 highflow can make 250-260 rwkw on a 25
a GCG vg highflow can make up to 270-280 (i've done 310 rwkw with it).
I can't see how you could possibly want more power than that. My personal all round pick would be the 25 GCG highflow