"GRS" - gender reassignment surgery - has a very broad, vague meaning these days. For most TIMs nowadays, especially the adult ones, "GRS" means surgeries that have nothing to do with the genitals: getting sacs of gel or fluid implanted onto their chests, facial feminization surgery, tracheal shaves, fat injections, liposuction, body contouring and so on.

Also, if he had his testes removed, he could take exogenous T just as easily as he could continue to take cross-sex hormones.

As for having to stay on exogenous hormones "for life" - I'm not sure that is the case for all men, though I don't really know. I know older and elderly men with "low T" or who are "hypogonadal" for one reason or another, and they're not taking either exogenous T or cross-sex hormones.

Lots of women have to have their/our gonads removed for medical reasons, but it is not customary for HRT to be prescribed for us for life coz for the past 20 years, the medical profession has been acutely aware of the health risks of true hormone replacement therapy (meaning replacing the sex hormones one's own body would normally take) in women. Usually, the HRT of women who've had gonadectomies is cut off by or at the age menopause would occur (circa 50).