Use a monstermaker for that. Not only can it spawn monsters, it can spawn waeapons, ammo and items and perhaps even more.
In the Monster Type field you type in the entity name of the weapon you wish to spawn (ie: weapon_gauss).
To make it repeatable you can try this:
- Each time the mm is triggered (only if the weapon has been picked up)
- Have it automatically spawn a new weapon when the player picks up the weapon already spawned.
You need to play with the other properties and flags because i don't remember exactly how its done.