Gretsch scale length is normally one of two scale lengths. 25.5” for the large bodied (17” wide) models like the Falcon. The others have a 24.6” scale. As McCreed says, the kit page doesn’t state the kit’s scale length, but I expect it should be either one of the normal 24.75” or 25.5” scales the kits come in. I doubt it will be 24.6” if it’s based on a smaller Gretsch.

As McCreed says, measure from the nut to the middle of the 12th fret to check the neck’s scale length. With the neck in place, you should measure double that length (maybe a couple of mm longer) to the high E bridge position. If so, at least the neck is the right scale length for the body. (The ES-3 neck has a 24.75” scale). If there’s a mismatch, then the bridge position will be about 10mm further away from the nut than it should.