It seems you need Science Lab (i.e. Promo II) to research Gauss Rifle, and you can't use it without researching it first. It can be useful to unlock Mass Driver Weapons, but that is only possible only after you've interrogated a non-underwater alien, so it's usually something to do for 1998. The biggest early/mid-game benefits of gauss weapons are Mass Driver/Gauss Cannon (a great craft weapon) and possibly the same for tanks. Later on gauss weapons can be useful for one-hit killing heavily armored enemies like heavy MIBs (if you don't have anything better) or sectopods, but this is really feasible only after you have obtained heavy armors.
Depending on the playstyle and RNG you could get those elsewhere as well. If you become tight on money, you could sell it, but at least now on, I'd try to keep it.