If you're done with kids you may not need to bother with HCG since that's what protects fertility, but it also has other benefits. The one most talked about is preventing testicular atrophy. You take testosterone, what little testosterone you were making ceases to be produced, it doesn't need to be since you're getting it from an outside source, and as a result the testicles shrink a little, mostly just lose some of their fullness. They don't vanish. And most lowt patients who've had low t for awhile have normally already experienced gradual atrophy and may not have noticed it for a while. Anyway, testosterone atrophy itself is not a life threatening problem and presents no damage to the body. Some people just don't like it. Another benefit, the best on IMO for HCG is it can help maintain stable blood levels of testosterone in between testosterone injections. It keeps your natural testosterone production going while taking testosterone giving you small burst of natural t each week. So your levels don't go up and down as much and remain more stable.

I was a fan of HCG in TRT for a long time but now I'm a bit hit or miss about it. The most important factor in TRT is getting testosterone and estradiol levels right (testicle size can't be put into the important category) and some men will find it becomes increasingly difficult to control estrogen when using HCG as it like testosterone aromatizes. Any aromatase acitivity can be combated through AI's but long term you want to think use as little AI's as possible and simply get to a good balanced number. I did the Test and HCG injections for several years, worked fine. Then for a year my Estradiol levels started going all over the place. I've since dropped the HCG and now use only testosterone and a very low dose of anastrozole. Currently it's 60mg of testosterone cypionate 2x/wk injected SubQ and one 0.25mg capsule of anastrozole after the weeks first injection. E2 levels have come down nicely and I hope to do away with the AI in the next month or so.

Your weight gain, it's not all muscle, some will be water weight due to the testosterone conversion. For now my advice would be to stay the course and have blood work done at the 6wk mark.
At minimum the test should include:
Free & Total Testosterone
Sensitive Estradiol
CBC w/diff
And PSA if it was not included on the first one. If it was included on the first one you can hold off on that one until later on. It's good to have the PSA checked every 6mo while on treatment.

Shoot back if you have more questions and let us know how the doctor visit goes.