The Archgun Deployer is a piece of gear that you can get in Warframe that will allow you to use your Archgun in a standard mission. The Archgun can normally only be used in Archwin modes, but this will enable you to equipped while on the ground in normal missions.
To get the Archgun Deployer, you will need to complete the third mission in the Profit Taker Heist. To take part in these missions, you will need to reach the rank of Old Mate with Solaris United, which can be done by completing bounties for Eudico at Fortuna.
Once you reach the rank of Old Mate, you will have access to the backroom in Fortuna and can speak to Eudico there to find out there plans to take out the Profit-Taker.
Play through all the available missions until you finish the third one, then the Archgun Deployer will be delivered to your inbox. To use it, you need to equip it to your Gear Wheel, which can be done in your Arsenal on the Orbiter, or through the menus. You will also need to go to the Vehicles tab in the Arsenal, and equip which Archgun you wish to use with it in the Heavy Weapon slot.
You will also need a Gravimag to attach to the Archgun itself. You can get the blueprints for these in the Tenno Research Lab in the Clan Dojo.
When you are in a mission, you can then click on the Archgun Deployer in your Gear Wheel, and it will drop out of the sky and into your hands. You will be able to use it for a maximum of five minutes, or until it runs out of ammunition.