If you're using the Firearm Mod rules in Arsenal, here's how it goes:
1.) The Smartgun has Gas Vent 2 and Sound Suppressor as built in mods that take up no slots (Arsenal 148). You cannot put on a different Gas Vent system either as a mod or accessory (Arsenal 148).
2.) You can take out the Gas Vent 2 with an Armorer check with a threshold of 2 or 1/2 the threshold required to put in Gas Vent 2 (Arsenal 129). This gives you back no slots (Arsenal 148).
3.) With the Gas Vent 2 removed, you can add in the gas vent 3 barrel accessory or the gas vent 3 modification as you would with any other Gas Ventless firearm.
Oddly, you cannot remove the gas vent accessory at all once you put it on your gun (SR4A 322) so keep that in mind.