I’ll take a stab at it, with a caveat of I currently own a G&L SB-2 Tribute, not a JB-2, and I have had 2 Blacktop Precisions over time, but have sold both of them.
It would be good to know what specifically you are looking for comparison: tone, neck feel, build quality, etc. , as well as if you are asking about a USA or Tribute G&L. Can you outline what you are looking for in the comparison?
Generally, there is more that is different about the Blacktop than my G&L than the same, with the only “similarity” being the tone from the pickups of each are very aggressive. I would describe both as “punch you in the face” if you want them too.
Beyond that, the neck shape/size/finish are very different, as well as the overall construction. IMO, I think the G&L is a better instrument overall. As I said, I still have the G&L, and not the Blacktops, FWIW.
As for the JB-2, it is intended to be a Jazz bass (2 single coil pickups, vs the humbuckers on the Blacktop P) with a more compact-Precision style body. So tone-wise that will be the major difference, and build wise, slightly smaller and potentially lighter.
Hope that helps.