The term "gaspipe" most generally refers to low-end straight gauge tubing of any sort. And yes, it's heavier, but often it'll only make a difference of 2-3 pounds overall, when compared to a lightweight steel frameset. It's the rest of the componentry that makes the bulk of the weight, and you were prolly feeling the difference of a more modern drivetrain on that Sears. Remember, a 3-speed hub is pretty heavy. So are steel cottered cranks, sprung saddles, etc.