Perhaps tpatke knows that using the term GS would be much harder to parse for folk that have not worked there. I work for FWE, by the way.

I can imagine that, still, how come "Goldman's" with the 's?

https://en.wikipedia.org/wiki/Goldman_Sachs

It isn't:

https://en.wikipedia.org/wiki/McDonald%27s

Likely British usage. We pluralize company names in many cases: Goldmans, Tescos, etc, and refer to them plurally (so, "Goldmans are seeking to ruin Sergey" vs. "Goldman is seeking..."). Apostrophe is probably wrong :)

So would a Briton say "I used to work at Goldmans" if the company is called Goldman Sachs?

Yes, exactly. Same with Lehmans, Merrills, etc. Mostly companies named after people.

In southeast Michigan its also standard for people to append an `s` to the end of most stores.

Kroger -> Krogers Meijer -> Meijers

etc. I never noticed it until I went on a linguistics bender on Wikipedia where it was noted.