I've never had to wait more than 10 minutes for gas at Costco, even mid day. But I do try to never get gas on a weekend and usually go in the morning or evening. The Costco gas near me is open until 9:30 on weeknights. I drive up anytime between 6pm and 9pm and there are maybe four people in the whole station.
As far as GasBuddy, I used it for a while many years ago. But I never really found it all that useful. Sure I found it fun to try to update prices when I could. But then I found myself paying more attention tying to update prices than paying attention to the road. So in the end I just stopped using it. So no I don't have their card and I wouldn't use one. Just like I don't like to go to stores where I get a discount off some items by joining their club.