I found all the gym leaders (at least the first five, as I only have five badges) pretty easy, probably because I caught every Pokemon possible in Leaf Green before facing them. Here's what I can say about the Gym Leaders:
--If you have a Butterfree and/or Mankey, Brock is easy. If you don't, he's the hardest one out there.
--Misty is so easy, unless you don't have any Grass or Electric Pokemon or all your Pokemon are below level 15.
--Lt. Surge is in my opinion the hardest gym leader out of the ones described here. I fought against him with an all-Ground team and still found it hard. Raichu spends the beginning of the battle using Double Team six times (or at least he did when my Sandslash was out), so after a while he gets pretty hard to hit. Sure, Voltorb and Pikachu are easy to beat, but Raichu makes the battle hard.
--Erika is kind of challenging but not too bad. If you have a Psychic, Flying, or Fire-type at at least level 30, you should be OK. (Though I found Erika challenging even though the Pokemon I used against her were Butterfree, Weepinbell, and Beedrill. My Butterfree nearly lost to her Vileplume >.<)
--Koga is soooooo easy. All you need is a level 35 Kadabra and you're all set. (My level 35 Kadabra KO'd his Koffing and Weezing in one Psychic attack--even Weezing has low Special Defense.) You might want to use a Graveler or something against Muk to be on the safe side--or at least that's what I did.
I don't know what Sabrina, Blaine, and Giovanni are like... but I am currently worried about Sabrina now... I know that she was tough to beat in Yellow, so I don't know...