Help overriding default gateway using packet marking

Hi guys!
I’ve seen this many years ago, during MilroTik training courses, now I can’t find examples…
I need to override default gateway using packet marking:

RB750:
eth1 = 192.168.1.2/24, connected to adsl router (main internet access) 192.168.1.1/24
eth2 = 192.168.10.2/24, connected to GPRS router for backup acces to RUT750 192.168.10.1/24
default gateway is 192.168.1.1/24

I can’t set masquerade on GPRS router (due not my router, I have no access on it), so when I access to RB750 from public IP, pockets come from eth2 but outgoing through eth1.
How can I mark eth2 packets to turn back through eth2 regardless default gateway says “go back through eth1”?
Thanks.