Is your 3CX hosted, or local? That is going to make a big difference as gateways are not supported unless local to the server, unless you can use VPN.
If you are only looking to accommodate a single PSTN line, and don't expect that to ever increase, you might try to find a linksys/Cicso SPA-3102, or even the older SPA-3000. They will do a single line, but require manual provisioning, which while not difficult, is not something everyone wants to try. Both models are no longer made, but can be found on-line. If you do pick one up, the forum has a number of posts on configuration.