I don't think you could have low gravity be at the center of any reasonable action. That said, perhaps you could use it cleverly at a very specific point of the map so that enemy Soldiers, Snipers, and Engies could easily defend a section of the Payload's track because it goes through a brief low-grav section. In that scenario, BLU would need to go around that part of the track (using one of multiple flanks) and clear the room ahead before returning to the track to push the cart through the low-grav section. That would be hard to implement, but I could potentially see it working if it was executed really well (and, of course, underwent plenty of testing).
You could alternatively maybe have a small, clearly-delineated low-grav area that offered a potential flank route to BLU at the cost of being a risky route to take. That might not be the best, though, simply because that route would likely get a certain reputation.
Alternatively - and this is certainly the easiest way of implementing low-grav, if not necessarily the most creative - you could just make it a fun, mostly unobtrusive feature by having it in a mostly non-conflict area (think a one-way flank route with a one-way door at the end that's difficult to guard, or an "outdoors" section of one of the spawns like the one in Snowplow's first BLU spawn). It would be a fun touch to the map without messing with any of your map's (hopefully) strong gameplay.