speed issue

Hello - in a house, just had ATT Fiber 300 installed.  The gateway model is BGW320.  I have my desktop PC connected w/ LAN cable to the back of BGW320.  I have run the speedtest.net speed tests repeatedly over past couple days.  Best I've seen is ~220Mbps download - supposed to get 300.  I asked the tech while he was here about it and he said they don't trust the # you get on a PC cause it might be old or slow on its own.  He did something (i couldn't catch what it was exactly) but he implied that he ran some test at the gateway and it was closer to 300.

Anything I can do or check here?  Is there a way I can check the speed at the gateway myself?

Thanks