General

If me gaining weight was a crime, most of my coworkers, family, friends, random people would have been guilty of "aiding and abetting"...

cookies, donuts, pizza, candy bars,

I had been a skinny barista as a notable coffee shop in San Francisco . And rapidly gained weight after I quit. When I went in my old co workers and some old regulars were mouth dropped, and one of the younger girls asked if wanted someone to work out with

I don’t have a great answer to your question but I’ll relay an anecdote.
My last job was at a small business I worked at for ~10 years. When I started, I wore a medium-sized shirt and was fairly thin. By the time I left I had (deliberately) ballooned up to a 3XL shirt, and was definitely not thin. No one said anything directly UNTIL one day when a coworker (woman in her 70s) asked me a math question and my answer happened to be: “450.” She came back with: “What is that, your GOAL WEIGHT??”
This was only the second experience in my entire life being fat-shamed so it’s not like I had some clever response prepared or defenses built up. (She also said this right in front of a client.) I just sort of laughed nervously, but on the inside I was surprised and hurt since I thought this lady was kind and not the type to stoop so low.
In hindsight I probably would’ve gone the full-on comedy route, turning it into a male pregnancy joke or whatnot.

I'm fairly muscular and don't have the biggest belly just yet so I've been able to pass it off as me just bulking so far. I do have one coworker who's an absolute health nut and likes to comment whenever I eat anything unhealthy on my break, but we talk about lifting quite often so I'm usually able to shake his comments by telling him "I don't lift weights to lose weight, I lift weights so I can eat whatever I want and not have to diet". This usually gets him off my back for a while, until he sees me eating something else unhealthy and the cycle starts again. Also my job has been fairly good for my gain because my boss has a pretty big sweet tooth and finds an excuse to bring us donuts fairly often, but it seems none of my coworkers really have one so I'm often able to sneak in a couple extra donuts without anyone noticing. We'll see if I get any more comments when my gain becomes more noticable

Not from any work people because I have been fat most of the time the newer employees have been there.
When I started in the stables working horses I was 250 ish.
My boss noticed and said something to me about my weight gain and I told him the truth I was trying to get fat.
He didn't know what to say and said I don't get it but OK as long as you work and do your job.

That is a decent reaction from a Normie! Glad he did not judge you.

I think so.

My coworkers have been really positive about my weight and weight gain. It would be kinda weird if they didn't, since some of them are also slightly overweight.

And some would even say that they are borderline encouraging. We usually have every month a "pizzaday" when everyone orders for lunch a pizza or some other takeaway food. There is also quite occasionally some chocolates, cookies, or buns at the coffee room so there is plenty of snacks and treats available.

Clients are allright. They are really polite so I think they would never comment anything. Maybe once or twice I have had a weird look from a client, but nothing else has happened.