Something else to consider for anyone looking to gain weight, is that your training should also reflect this desire to gain weight. Eating at a caloric surplus, even if it's pretty clean, and not doing work that will stimulate a lot of hypertrophy, means you won't gain much muscle.
Hypertrophy work is more often than not a lot of volume. Look at traditional bodybuilding routines or early season PL/WL stuff. You'll likely see lots of sets of 10.
There's not anything magical about sets of 10, but it's a nice sweetspot that let's you build up a decent amount of fatigue, which is the driver for hypertrophy, without having your CNS fatigue tap out first. Studies have shown you can get just as much hypertrophy from any set/rep scheme, as long as the fatigue is the same. Building up a lot of muscular fatigue with heavy singles is just way too taxing (look up the Bulgarian weightlifting program from Ivan Abajiev if you want to see what volume made up of only max attempts looks like...we are talking max front squat, snatch, and clean and jerk *every day* indefinitely)
So if you gotta accumulate fatigue and volume from gym work, this will most definitely interfere with on the bike work, which means it's something best left for the off-season or base season.
Nothing stops you from doing a volume heavy gym session once a week in season, but it definitely has the potential take away from whatever you're doing in season.
Also worth considering that lots of volume gym work is really a lot of moving pretty slowly, which is teaching your muscles to contract slowly, which is very much not like pedaling at 120 or 140 RPM. It's also important you take the new muscle you gain and transform it into speed later. Hopefully this would be addressed through proper periodization.