What tool can I use to tighten SRAM GXP bottom bracket without removing the cranks?
The short answer? No.

Threaded GXP uses the Hollowtech II style dimples for tightening, these aren't deep enough for a C-Spanner style or for a wrench the has less than 180 degrees of contact.

Even if a tool existed, most chainrings are going to get in the way, my 1X is GXP and there's not a lot of room, a double or triple is going to make this worse.

And finally, GXP works by clamping the non-driveside bearing between a flange on the spindle and the back of the crank arm. There's no preload adjustment, so tighten it down and leave it alone.

PS. What style is your eccentric BB?