If you're a hard gainer and that's something you care about, I assume you're doing mostly hypertrophy. You should do exactly the amount of low-impact cardio that will improve the amount of volume you can tolerate/recover from, and no more. More than the minimum effective dose for recovery improvement will consume extra calories, and you need to eat like it's a war.
The three drivers of hypertrophy are
1) Mechanical tension as driven by intensity and volume (although even loaded stretching has been shown to increase hypertrophy, because it puts mechanical loading on the muscle).
2) Muscle damage, as involved in eccentric reps, sets to failure, or controlled cadence
3) Metabolite accumulation, as involved in extremely high rep sets, blood flow restriction, and sets to failure
So don't do any cardio that decreases your ability to do those things, and instead aim to just improve your recovery so you can add more sets over time. Slowly increase your caloric input, which means tracking it in the first place, so that you are actually eating more but not just getting fat.