I have also never gotten an email like that. I would be concerned that it might be a phishing scheme. Type in their website address (don't use links provided in the email), log in, and check your account info. Frankly, I never knew they charged for just being a member. I've never actually bought or sold anything on Gunbroker.
They don't charge for being a member. I contacted GB and they kept telling me I was being charged for being a seller. I never sold, nor attempted to sell, anything on GB. All I ever did was look and twice I bid on items I didn't win. Somehow they had it in their system that I became a seller when I did not. I asked them to tell me what I sold or attempted to sell. They couldn't tell me. I did that twice and they corrected the problem twice. The third time they got a little upset when I asked them why this kept happening and when they were going to fix their problem. I guess it's easier to lose a member rather than fix the problem. I tried, they didn't.