I'm pretty sure the Gauntlet barrel is choked. It's designed for pellets, and all mass produced PCP pellet rifles use choked barrels. But if you want to be sure, then slug your barrel. Take a pellet and push it into the breach end of your barrel. Then take a cleaning rod and manually push it through. At first you will have resistance. Then as the pellet conforms to the shape of the barrel, you will feel the resistance ease up. If the barrel is choked, you will feel the resistance on the pellet increase in the last 1.5 to 2" of the barrel. But I'm 99% sure the barrel in question is choked.
But the gun is also limited by the regulator. It's a Ninja reg with a preset reg setting. It's set for pellets, and the plenum is too small to send slugs downrange with decent velocities and stability. Ninja makes high pressure regs that can handle 4500 psi on the bottle side and have a set regulated pressure of 2800-3000psi. Not sure if it's compatible with the existing config on the Gauntlet, but if it is, that would be an upgrade that could potentially allow for slugs. But then you have your next hurdle.... If you upgrade the reg, can the o-rings in the vale assembly handle the higher pressures? And can the hammer spring generate enough power to break the seal on the valve assembly with the higher pressure of the regulator behind it?