I've been commuting from the Philly area for about a year and a half now. I can't speak to any other Gateways, but PHL is pretty easy. Five days out of the week there are roughly eight flights to SDF (and probably about just as many coming back). Currently there are no flights between the two on Sunday and Monday. However, AA has 3-5 directs to and from that you can use as a backup if you need to get to work.
As far as booking goes, the process is as easy as selecting your dates and flight numbers and hitting submit on your iPad/iPhone/laptop. As a UPS crewmember you can book 12 days out. From what I've seen, no one is really booking that early for a jumpseat between PHL and SDF and vice versa. Honestly, I don't think anyone in the system is booking any jumpseats that early unless you're trying to catch a ride to ANC.
The one good thing about PHL is that it's a big Gateway for the jets bouncing around the east coast and the ones going across the pond to Europe. As a commuter on any fleet but the 74 (as of right now, they were scheduled in PHL for peak), you can find trips with layovers in PHL and trips that start and/or stop with a commercial to/from PHL. That's pretty big for QOL since you're getting extra paid days at home. As a new hire and depending on what fleet you end up on, I wouldn't count of getting those right away, but they are out there and you should be able to hold those after a while based on the movement we're currently experiencing.
Obviously, things are subject to change at any time and without any notice, but those are my observations over my short period of time here.
Any other specific questions, shoot me a message.
Turk