Yeah, you need to fill the space horizontally, with a rich, full sound and lotsa mids, not vertically jumping octaves or whatever. Those overtones are there already. Longer, fuller, ringing notes to fill the space helps, and, as mentioned, a little crunch can help provide a more full sound. When the gp goes off to solo, the mix changes, so you have to change how you're sitting in that mix, by a combination of technique, note choice, and tone, since a tone that sits behind the guitar and does not crowd its sonic space is not the same tone you want when you are trying to fill that space.