From each point in your network do a speed test.

Http://speed.cloudflare.com is excellent

Do these tests with wired connections.

For gaming, the thing you really care about is latency and packet loss, even if your total bandwidth is lower, it shouldn't impact gaming. Especially at fiber speeds, if you're only getting 300 instead of 1,000 megabytes per second you will have no problems gaming

If you see packet loss and high latency on your wired connections, there's probably interference, the cable might be running parallel to a power cable, it might be damaged, it might be poorly terminated.

Ethernet cable is really cheap, you can just run extra cable. On the floor even and tape it in the corners of the walls. You can fish it through the wall cavities.

If that's not an option, you can use powerline ethernet, which will not be as good, but it will use your power lines, so you won't have to run cables.

If you're still having problems, please describe your network and detail to us, including the kind of cables you're using example:

Fiber-isp router-cat 5a cable to living room-switch in living room-cat 5a cable-gaming computer.