Gran Premio is not a lower end Univega.

The stem diameter is going to be either 26.0 or 25.4 mm. Measure the handlebar with a caliper where the stem goes over it. A ruler is not good enough. If your calipers are telling you 26.0 and you know how to use them, that is what they are. You need resolution to at least +/-.1 mm

The length is measured from the center of the stem bolt to the middle of the handlebar. A ruler is normally good enough.