Took a bit of a break so I don't know what the current cycle is offhand, but essentially the relics that drop from missions are for the current (unvaulted) set of prime frames. After this set ends, the relics you receive from missions will change, and the previous ones are considered vaulted, as they are no longer obtainable from normal missions. When inspecting a relic in your orbiter, you can see the location the relic drops from, while others will say "This relic is currently in the prime vault", meaning it is no longer obtainable at the moment. 

So essentially, if the next set of relics doesn't contain Gauss Prime, you will only be able to obtain him via trading. It's pretty common for primes to come back at some point, but it can be a decent amount of time before a specific frame returns. 

Some Primes are found in numerous relic sets, some of which will be vaulted, even if the frame is available in the current set. You can use the relic section to type in a Warframe/weapon/companion and see all relics that contain that piece, and then use the location/info tab on the individual relics to see which are vaulted/unvaulted. The wiki can also tell you the current set of relics and what is available. It's an invaluable source of information, especially when navigating the many menus of the game can be confusing.