Plain gauge means the tubing wall thickness is consistent along its entire length. It's not butted - or thinned in the middle - like a lighter and more expensive tube would be.
Tange is a quality tubing manufacturer, so even their low-end product is very decent, in my opinion. The No.5 tubing isn't bad, but it's not great either. It's very solid, but a bit heavy. I wouldn't spend a lot of money to build it up with racing-level componentry if I were you, but it's worth keeping, I think.