There are a lot of factors to this, some of it may be the rib, some of it may be that he is a trainer on NXT so it's a way of sending a message to the talent there to "keep it clean" when it comes to what you put in to your body. He would wear a T-Shirt cos they want to sell them... pure and simple, if he's wearing the shirt people will buy it. He's hardly in the Ric Flair/Last Nitro situation to have needed it. By his last match Flair was clearly old, but he was in great shape and that too was a major part of the commentary of his matches.
It's VERY important to note Gunn was part of the post "roid scandal" WWF when he came in, so while he may have potentially partaken a bit during the DX days to improve his physique, most of the time he was plain old Smokin' Billy Gunn he didn't have that look at all, he was part of a locker room tested and fired/suspended if they screwed up for those first few years. With guys like Dustin, Billy, Regal to a lesser extent they messed with these things far less than "recreational substances"... guys like Sean Waltman have clearly over indulged, guys like Billy may have partaken, but generally if booze is your poison, you come out far better and Billy, like JBL, Simmons and Taker were part of that "drinking culture" as opposed to the drugs culture in the WWF mid 90's...
Gimmick wise there is an element of "growing old disgracefully" to it as well in Gunn's case, that he's still alluding to his "suck it" line and playing up to the old images of B.A. Billy and Mr. Ass somewhat so that is something they're gonna put over in the commentary that he is older than most of the roster but still "raising hell". Just as they would if it were Stone Cold...
Sometimes people just age well and when wrestlers do it's pretty rare but also valuable and worth putting over as an example to kids, their folks and aspiring wrestlers.
Guys like the Rock, Jericho are in their 40's and you don't imagine them aging much between now and 50. Their schedules are light in the ring but they still train as if their next match is tomorrow. It reminds me of Dennis Stamp on Beyond The Mat, a guy who clearly had no real career to speak of but for his age was in phenomenal shape and he said..."You never know when someone's gonna call..."
Taker is not aging well at all, that may be down to the beatings over the years but it's more down to the not keeping in "great shape" a full time schedule demands... most near 50 year olds could manage to look half good for one or two nights a year, but Taker even struggles with that now. He knows Vince isn't gonna call without serious lead time, he knows that his opponent will go to his for 2 weeks and they will design the match... the other 8 months or so of the year he could be getting seriously fat for all we know.
Jake looked better than he ever has since he left the WWF and it'll be interesting if Hall is put in the HOF (guess they figure do it while he's half lucid and DDP will be there to keep both he and Jake in line) what condition he shows up in... he can't fail but to be the opposite end of the Billy coin really... some people are gonna be stunned he is younger than Jake and a similar age to Gunn. Even Lawler doesn't really look his age, maybe a bit closer now the heart attack hit... hell Vince is only now starting to look like his age and for the same reason as a lot of the others... once he's off screen he doesn't NEED to train as hard as before... when he was showing up in the tank top vests and doing covers of magazines, he had to look good (and very likely juice) now it'd be about fitness and general well being as he's covered in a suit all the time... he's always gonna be a "big guy" but once the intensity and exercise stop, you start aging quicker as it's the body's way of healing.
One guy who looked in AMAZING shape last year when he showed up was Bob Backlund... For a guy his age he was very impressive although it was a short appearance. I was even hoping he'd get a US title match against Cesaro or something. If that really is what the CM Punk/D Bry kind of lifestyle can do for guys then it was a big advert to the younger guys, but in a small understated way.
In terms of how they are putting over ring work it's not much different to what they have done with Goldust in recent months and with Regal for the last several years of his career. Once someone hits their mid 40's if they are "aging like wine" then it is something to be praised somewhat as it's not easy to keep at a level or improve that level once you hit that age, either in terms of physicality or appearance. Look at Shawn, now look at Billy Gunn... Shawn is suffering for his years of drug abuse and those 4 years of inactivity during his prime that he arguably didn't need to take off... the superlatives would flow if he came back and was able to go but they would have a hard time calling him a Boy Toy now though or even the Heartbreak Kid is somewhat wrong...