Upgraded to Gig a little while back, but I'm getting worse speeds than when I was on Blast 250.
I have a Netgear C7800 modem. It might be important to note that there is still a splitter outside (looks like a 3-way), since the house was wired for cable TV. Of course, I've cut the cord and stream everything, so Gig is pretty important.
I've attached a screen cap of my modem status page. The computer I'm testing from is hardwired to the router via a Cat-6 cable. However, right after I upgraded, I was even getting > 400Mb on WiFi.
FWIW, it appears that the modem is not locking on to the DOCSIS 3.1 / OFDM downstream channel (channel 32) which provides gig speeds. Have you tried rebooting / resetting the modem ? Also, it may be provisioned incorrectly.