The gym leader levels I can see being: {(gym number) x (10) + 5} for gyms 1 to 3, but after that I see them adding on about 8 lvls per gym after that. So gym 1 to 8 in order (in my mind and do take this with "a grain of salt" as said earlier) are 15, 25, 35, 43, 51, 59, 67, 75. This would make more sense seeing that most people say to be at least lvl 75 for all your pokes to face gym 8 in both regions. For the e4 I see the jump from 8th gym being a significant leap of 10 to 15 levels, leaving the e4 at 85-90 for the levels. This kinda makes me scratch my head though, seeing that all of my Pokemon were EV trained and above level 84 for the kanto e4, yet it was still a significant challenge. I don't know if the devs coded them to be EV trained, or if they're just above the normal levels of what we think because of the lack of EVs. Boss Pokemon or programmed in the game at lvl 120 and the only reason that I can see for that is because of the lack of EV training on those Pokemon. That being said, if you take into account that though maybe the levels of the gyms are more like 15, 25, 35, 44, 53, 62, 71, and 80 for the lack of EVs and the e4 levels being around 95 to 100 also do to the lack of EVs. That all being said though, the levels your Pokemon should be when facing the gyms in order should be around 13, 20, 30, 38, 45, 54, 65, and 74. For the kanto e4 a good team of lvl 85 pokes should do nicely and for johto e4 a team of 90+ should do well too.
Regards,
Kxrizmuh