You don't earn MGP by farming GATES and so, you get MGP by completing weekly challenges.
- First the challenge logs. If you take out the too grindy stuff (To the races II, Victory Lap II and Kiwami) that's a total of 71k MGP
- Then by completing the challenge log you will : races 3 time (3x1k MGP) and do 5 GATES (5x5k MGP)
- By starting and loosing 15 Verminion battle during a tournement you'll get some MGP as well, ~35k weekly.
- Fashion Report grants you 60k MGP weekly
- Mini Cactpot ~7x500=3k5 MGP
- Mega Cactpot ~3000 MGP weekly
Except for the challenge log, all the MGP gain can be increased by 15% with
Gold Saucer VIP Card. Completing all those takes maximum 2h a week if you take your time.
TOTAL = 71k + 1,15*(3k+25k+35k+60k+3k5+3k) ~= 220k / week and I did pessimistic estimations. So to go from 0 to 10M MGP it takes around 45 weeks. less than a year. I think that's acceptable.
Example : I was cap'd before the mount. I spent the 4M MGP for the mount so 6M left, and I'm already standing at 7M5.