It should be safe to go out to it. Just don't stand in front of the nozzle or the little nut on the side of the nozzle - that's the relief valve. You can try to make sure it's turned off.
Can you walk through exactly the steps you took? I assume the cylinder was closed when you attached the regulator, and then when you opened the cylinder you heard something hissing. Well, my guess is it wasn't the cylinder that was hissing, it was the relief valve on the regulator. It's designed as a CO2 regulator, which means that the relief valve on the reg (not the one on the cylinder) has a much lower cracking pressure than 3000 psi, even though the gauge shows up to 3000 psi.
So, a couple questions - one, did you remove the regulator, and if you did, does the high pressure side now show zero?
Two, did you just get the cylinder refilled, within the last hour or so? if so, the CO2 and cylinder will still be warming up, and pressure will be increasing. I would definitely leave it outside for a while in this case.
If the high pressure gauge on your regulator is back to zero, then the gauge is *probably* working correctly, and the pressure in the cylinder is way too high, meaning the cylinder was overfilled. It's not safe to use until it's at a lower level. Unfortunately, I think your best option *in this case* is to wait for it to warm up and see if it vents, and then, if it doesn't, go outside and manually open the cylinder valve and allow the CO2 to escape. Then take the cylinder back to the refill place, give them hell for putting your safety at risk, and demand a free refill. Or a refund, and take it somewhere else to fill.