Gold member just means they joined with and pledged a minimum $25,000 annually. The Bitcoin Foundation doesn't do any of the other things you suggested such as certify, endorse, or guarantee any company.
This is what I suspected, once I gave it some thought.
However, when I first glanced at it, I thought it meant they were on the board of the Bitcoin Foundation or part of an executive body of that group, or were endorsed by the Bitcoin Foundation, or something similar.
I'm sure others think the same currently.
In actuality, it just means as you pointed out that they donated money to the Foundation.