Interesting idea, but, at least on release, 5G won't work great for gaming. (That might change as hardware is refined over time, much like with Wi-Fi.) To determine why, it's important to note that "online gaming" will likely be a bit ambiguous by the time 5G is widely available. There are two main types of gaming that could be categorized as online gaming:
- Multiplayer gaming, where the multiplayer aspect is achieved via interconnection across the internet.
- Cloud gaming, where the game actually runs on a computer in a datacenter somewhere, and the hardware is essentially rented by the players. This isn't too common at the moment, and critics are skeptical of its long-term viability (will it just be a fad?), but a bunch of big-name companies are investing heavily in it, with the most recent announcement coming from Google in the form of Stadia.
5G should be usable for the first category under optimal conditions, but it won't be ideal. The main advantage of 5G is that it's cheaper to deploy to any given home than a traditional wired connection. It's a little more unstable than a proper wired connection; for example, it's theoretically susceptible to changes in weather. This can result in high packet loss and jitter, and that's exactly what you want to avoid while gaming online, especially competitively. In a proper installation, it should work fairly well, and the problems should be minimal, but most people won't be deliberately searching for 5G for the purpose of online gaming. Gamers who are choosing their internet service based on their gaming interests generally have strong prejudices toward certain tech; even if 5G would be suitable for their purposes, it's not something they're going to be deliberately seeking, so they won't be googling "5G online gaming".
5G isn't really usable for the second category. Although 5G speeds can theoretically be fast enough, even the slightest instability (packet loss, jitter) will be extremely noticeable and annoying. For this reason, wireless networking of any kind is avoided like the plague for cloud gaming. (That's actually one of the main factors keeping cloud gaming from going mainstream: to get it to work well, you need a fast, stable, and usually fairly expensive internet connection, and all the networking in your house has to be perfect, down to the crimping on each wire. Since the main attraction of cloud gaming is its price, the need for expensive hardware and a decent amount of networking knowledge kind of defeats the purpose.)