Online games use shockingly little bandwidth, typically 50-200 kbps. A 5 Mbps connection can run any multiplayer game ever made. What gaming really needs is low ping, low jitter, and zero packet loss .
Upgrading from 100 Mbps to 1 Gbps changes nothing for game ping. The packet still has to travel the same physical distance. The only ways to lower ping meaningfully:
Downloading a 100 GB AAA game is the only thing that benefits from gigabit plans, it goes from 4 hours on 50 Mbps to 15 minutes on 1 Gbps. Once the install is done, ping rules.