There are several reasons one might not get 1gbps on their connection. I also have Gigablast and get 950 most of the time. Methods of speed tests on high-end connections matter. I don't use speedtest.net cause it seems to give lower speeds. Try using multiple speed test sites. I use dslreports.com and seems pretty good for my area.
Older computers (even if they have gigabit ethernet ports) can cap their CPU on fast data transfers which can limit speeds your particular computer can reach. This is more notable on any non i3/5/7 series of intel or non ryzan 3/5/7 cpu for AMD. If you're not running a current gen of cpu you "may" not be able to achieve full 1gpbs speeds (download).
If you're using your own router (especially the nighthawk Netgear routers) they have a bad habit of not getting 1gbps, you can try to unplug the router and plug directly in your modem (and then reboot to get your pc a valid ip). You'd be surprised with how many routers advertise gigabit but don't sport the hardware to actually get you those speeds.
Also as a final note, you will not get gigabit on wifi AC speeds, so if you're not on a wired connection, don't expect to get the full gigabit speeds because that's a technology limitations.
