There are basically 2 ways to overlay lines in gplot ...
As Cynthia mentioned, you can probably use "plot y*x=z" (where z is what you were using as the by variable).
Or, you can use something like "plot y1*x y2*x y3*x / overlay" - for this technique, you'd probably have to transpose your data, and it requires hard-coding all the (new) variable names in the plot statement. It's hard to get real specific without seeing the exact data, but basically you would have to transpose your data (proc transpose) so that you're basically turning the text values into variable names, with the x1 numeric values becoming the values of the new variables.
You'll probably want to go with Cynthia's technique, but just wanted to mention the 2nd alternative!