In Battlefleet Koronus , Table 1-3 (pg. 14) lists the craft ratings for various small craft.

However, the table lists Fury Interceptors, but not Gun-cutters. Looking at the statistics in Into the Storm , Gun-cutters are slower than Interceptors, and lack the void-capable missiles that Interceptors have.

My question is this: what would the craft rating for a squad of gun-cutters be? Or are gun-cutters not particularly useful for escorting starbombers?

It lists Gun-Cutters as Assault Craft. Not having the book with me, but from memory they can still escort Bomber Craft, and every on a 2 squadrons of Assault Craft add +5 rating, I think.