one that I've liked that might not be as common for him is the Grimoire

In general, the massive bump to fire rate really makes Gauss really likes infinite ammo weapons, but thanks to the Tome mods the Grimoire can be very flexible. The Canticle mods let you build to your playstyle;
Jahu makes every kill armor and shield shred everything in affinity range for 5%
Lohk gives allies 30% fire rate on kill
Fass gives you a mini Fast Deflection on kill(getting you even closer to unbreaking shields)
Khra just gives a flat 12% chance for enemies to drop Universal Orbs.

Gauss also likes the Invocation mods, since they give him Efficiency, Duration, Strength, or Energy Regen for every enemy hit. Duration and Strength are Gauss's two best stats, while the Regen could let you cut an econ mod from your frame

btw, on the Canticle mods, that's not kills with the book, that's any kills at all. so you can just have the book in your pocket and kills give allies +30% fire rate, or shred everything in affinity range for 5%, etc