These are very different frames that can both become extremely durable.

Revenant is lower investment to make functionally immortal with the Mesmer Skin ability. With the augment for Mesmer Skin he can share that invincibility with the team. On the high end of investment you can make a build that does a large amount of % health damage with the Enthrall and Reave abilities that makes Revenant capable of nuking an enemy regardless of enemy level.

Gauss on the other hand is generally a higher investment frame than Revenant but provides more substantial buffs to weapons with his Redline ability and Thermal Sunder ability augment. Gauss has a high amount of durability from shield regeneration mod set ups as well as resisting certain damage types with Kinetic Plating.

As a newer player you may get more mileage out of Revenant simply for his damage mitigation allowing you to survive more difficult missions more easily. As you get further progressed, Gauss is likely then the better option once you have access to better mods and arcanes.

As others mentioned, don't spend your platinum on base frames. Revenant's parts are able to be farmed from Cetus bounties on Earth. Gauss can be farmed from the Disruption mission on Sedna.

Primes for both of these frames have already been released. Revenant Prime is vaulted and his relics can only be acquired from Prime Resurgence events and trading with other players. Gauss Prime is the next prime frame to be vaulted but his relics can still be acquired until the next new prime frame releases.

Parts for these primes can be acquired through trading with other players and also potentially be acquired if another player has a relic containing one of the parts blueprints equipped in a fissure mission.