The more I play Guardian, the more I find myself turning to Lightning Gun as the best upgrade path for multiplayer Guardians. It is so good against the Expert chaos bag that if I don't build for it, I end up wishing I did. The +5 bonus means that with only modest further boosting you can crack the -8 token on every shot. The +2 damage means that you have to spend fewer fight actions to kill things, which means fewer pulls from the chaos bag and more actions left over to do whatever you need to do.
Of course, Lightning Gun very much demands to be built around. You must have ways to reload it in your deck--Extra Ammunition and Custom Ammunition are your best in-faction options here, and both are worth it. I think Venturer is fine, and I might run one or two. But my first choice would be to play the ammo events and use my ally slot elsewhere.
I recommend supplementing Lightning Gun with extra-damage effects--Beat Cop II and Vicious Blow are the most important. These save you ammo, skill tests, and actions by letting you kill 4-health (sometimes even 5-health) enemies in one action. Beat Cop II will also spare you from having to use Lightning Gun on something dinky like an Acolyte or Whippoorwill.
You were probably running Emergency Cache in most of your Guardian builds anyway, but it's as necessary here as anywhere else.
One support item I generally omit is Bandolier. I don't think there's a huge advantage to having the third hand slot, and Bandolier + Lightning Gun + another hand item is needlessly expensive. That said, you do always want additional assets in play to go with Lightning Gun. Otherwise you'll risk losing your Lightning Gun to stuff like Crypt Chill and Pushed into the Beyond, and that would just be disastrous. Just use assets that don't take up hand slots.
I would skip "Eat lead!" most of the time--the guaranteed hit is nice, but spending extra ammo and 2 XP isn't. Maybe if someone is packing Double or Nothing this becomes better, but it's probably still a low priority.
Make no mistake, this is a very greedy card. Most of your resources and much of your deck space will have to be devoted to it. It will take up a ton of XP, in a faction that is entirely devoid of XP acceleration. Attractive XP options like Stand Together will have to be significantly delayed, if you ever get to them at all.
But the upside is that a properly supported Lightning Gun will be outstanding at your primary task of blowing up enemies. You will do that far more efficiently and reliably than you otherwise could. Plus, when it comes time for a boss fight, you'll be ready.