after clearing all 8 across 4 saves, and using all three starters. the only gym leader i found consistently hard was elesa, electric to sort out oshawatt, and flying for snivy/tepig. as well as flame charge on the zebra.
the first gym is made trivial by the gift monkey
the second by the lvl 17+ throh/sawk in the nearby grass
the third can be a pain for water/grass starter. but bad movepools and low stats on his pokemon make him ok
fourth is a pain, as stated
fifth is fairly 'meh' never fought him and come out thinking he was exceptionally hard, or easy
sixth was a pain with snivy, but trivial with tepig (and rollout), or slash with oshawott
seventh was a pain with snivy (again) but fairly 'meh' with osha, trivial with tepig
i always had a haxorus in my team by the time i got to the eigth gym, so was very trivial too.
the only recurring theme was how snivy has not one gym where he is super effective against all pokemon (he's weak to 5 or 6 out of 8)
while tepig is strong vs 5 or 6 with the right moveset.
this surprised me, as they normally give the grass starter the type advantage to make up for the less then impressive movepool and stats