You need a DOCSIS 3.1 device to get gig speeds. Your SB6190 is a DOCSIS 3.0 device and won't be able to achieve those speeds. If you want gig speeds, you 'll have to upgrade your modem (see http://mydeviceinfo.xfinity.com). I don't know why they upgraded your account to gig, they should have known your modem wasn't the correct device.
BTW your signal levels are too high, you'll want to knock those down with an attenuator/pad, assuming you didn't install a signal amplifier.
Too much signal is just as bad as not enough. I'd suspect that your levels aren't far enough out of spec to be an issue though. If everything is working fine, then I'd leave it alone. If you do run into issues, consider knocking down the power levels.