I actually did the GPEN via OnDemand. I also did it after the OSCP. Nothing compares to the OSCP. The GPEN compliments the OSCP quite well. There is in fact some fact some amount of overlap with the OSCP providing better coverage in some areas.
The difference between the two is more than just laws

the GPEN covers a lot of the business aspects of doing a pen test. Topics such as defining scope, creating a get out of jail free card, defining the rules of engagement etc are covered. So the OSCP gives you that 'raw' skill. The GPEN will help 'refine' it

As mentioned above grab the practice test and gauge your readiness from that. The test costs $99. If you do decide to go the ondemand route then keep checking the website as ever so often there are promos that offer discounts of up to 25%. If a discount is not offered, then go out on a limb and ask for one.
The CEH........that was my first certification. When I did it I knew NOTHING about security. So given that you already have the OSCP getting the CEH should not be difficult.