How to Unlock Archetypes in Remnant 2?

The Gunslinger is one of the default starting classes in Remnant 2, and it can be unlocked by simply choosing it as your starter archetype if you pre-ordered the game. 
If you didn’t choose the Gunslinger as your starting archetype, you can unlock it later in the game by talking to Mudtooth in the Ward 13 container crane and exhausting all of his stories. After doing so, he will hand over the Worn Cylinder, which can be taken to Wallace to upgrade into the Iron Cylinder Engram and unlock the Gunslinger.
When you have unlocked the Gunslinger, you can also use it as your secondary archetype in a co-op session, as the class is great for close-range engagement. 

There are six other existing archetypes in Remnant 2, but they are more difficult to find and can only be unlocked by playing the game. 
The Medic can be unlocked by purchasing the Medic Pin from Dr. Norah in the Ward 13 hub, and upgrading it to the Caduceus Idol with Wallace. 

The Summoner is one of the most difficult to find, as it requires players to progress deep into one of the game’s worlds to reach Yaesha. Here, they will find a Bloodmoon Altar shop, which sells a Faded Grimoire for 15 Blood Moon Essence. Taking this item back to Wallace in Ward 13 and equipping it will give the player the Tome of the Bringer Engram that will unlock the Summoner archetype.

The Engineer can be unlocked by completing the Forgotten Prison area, where players will encounter a powerful creature called Tal’Ratha. Killing the monster will reveal a mysterious stone, which can be taken to Wallace to unlock the Engineer.