Well, this is how I rate the difficulty of the Sinnoh Gyms.
Oreburgh City Gym
Just make sure your Pokemon outspeeds Cranidos, so that it can't Headbutt flinch you.
Eterna City Gym
Do not ever let Gardenia's Turtwig set up Reflect, also Roserade isn't easy to defeat.
Hearthome City Gym
Fantina's Mismagius is annoying, with its STAB Shadow Ball.
Veilstone City Gym
Not that difficult, but Lucario can be rather annoying sometimes.
Pastoria City Gym
Your Pokemon should be a special attacker which outspeeds Floatzel and also carries something like a STAB Thunderbolt. Special attackers also do not mind the Attack drop from Gyarados's Intimidate and can KO it before it starts spamming Waterfall, which can prove deadly if not resisted. Also pack a Grass type to counter Quagsire.
Canalave City Gym
You need a special attacker with a STAB Flamethrower to hit Magneton and Steelix hard, and also something with an attack that is x4 Super-effective against Bastiodon.
Snowpoint City Gym
One of the hardest Gyms in Sinnoh in my opinion. Sneasel and Piloswine are not really threats but Abomasnow and Froslass are. You need to use a weather-inducing move to get rid of Hail once Abomasnow switches in, and KO it with a strong STAB Fire type attack. If Abomasnow manages to switch in and immediately get Hail up and you can't change the weather, watch out for Froslass coming up.
Its Blizzard, in Hail has a 100 Accuracy and also a chance of Freezing. Do not underestimate its Blizzard, or you'll regret it. Besides Blizzard, it also has a STAB Shadow Ball and also Psychic, which is also nothing to laugh at.
Just remember to pack a Fire type when facing Candice
Sunyshore City Gym
Volkner's not easy to beat. Besides having strong STAB Electric attacks, Luxray has access to Ice Fang, Raichu has Ice Punch if I remember correctly while Electivire also has access to Ice Punch. You need to beware even if you are using Garchomp, because if it's under-leveled, Electivire and Raichu will most likely outspeed and OHKO it with Ice Punch.
If Garchomp's is about 5 levels higher than Electivire and Raichu, considering a Neutral Nature(Hardy, Docile, Serious, Bashful, Quirky), it can outspeed Electivire comfortably and outright KO with a STAB Earthquake.
Also, beware of Jolteon's raw speed. Luxray is also not easy to take down. Garchomp's level needs to be higher than Jolteon if you even want to outspeed it.