Hi Jens,
I haven't worked with Human Resources standard, even in ABAP ECC world. I have had some problems with the RFC sender in the past (XI 3.0 and PI 7.0), i had to restart and to reactivate the channel with certain frecuency, the main problem was, as you have detected, the gateway situation, system orverload, network issues an so on.
Wouldn't that mean using the PIs very own gateway then? One couldn't get more near then that, right? :-) As said I'm on PI 7.4 AEX with a central adapter engine (no decentral adapter engines in place).
Yes, if i was you i would configure the gateway in the PI AEX.
May be you should wait to another SCN member with more experience using RFC sender channel in the 7.31 or 7.4 PI java single stack.
Regards and good luck.