Gen II
Falkner
7 - 9
Bugsy
13 - 16
Whitney
18 - 22
Morty
21 - 26
Chuck
26 - 30
Jasmine
32 - 35
Pryce
27 - 30
Clair
38 - 43
Gen IV
Falkner
10 - 13
Bugsy
15 - 18
Whitney
17 - 20
Morty
23 - 27
Chuck
29 - 31
Jasmine
32 - 35
Pryce
31 - 33
Clair
40 - 44
Johto Gym Leaders are notorious for difficult ace Pokemon (Scyther, Miltank, Gengar, Steelix, Kingdra), so quite a few of my recommended levels are above the level of the leader's ace. It may be tough to reach these levels due to Johto's awful level curve, but it will be well worth it to defeat these Pokemon more easily. You can absolutely be underlevelled, but quite a few ace Pokemon become significantly harder to defeat if you are.
Hope I helped!