On a typical gateway channel setup is controlled by a file called global_conf.json You’ll have to state things in terms of the IFs of the two front end sx125x radio chips and then the offsets of the baseband channels detected by the SX130x from those two IF’s.
If you look at the existing file in comparison to the band plan it implements and spend a few minutes studying the numbers you should be able to see how each channel is expressed as an offset from one of the IFs. Change those, change the offsets within a limit of about +/- 200 KHz and you can have a different channel plan.