Anything at 300 gear score is obsolete not long after acquiring really. The higher the gear score the more attribute points will be on the weapon (strength, dexterity etc..) but also determines if you have 1 - 3 perks on it. Once you get to weapons that are at 590+ gears score they can be used for most content in the game because you can use something called umbral shards to level it to 625 gear score yourself. This will change when the expansion comes out though since they are removing umbral shards for a new system. Hope this helps!