While many SIP/GSM/PRI/PSTN gateways will work with 3CX, they just haven't been tested, or have a provisioning template available. This all means that if you choose to purchase one that is not supported, you are on your own. It doesn't mean that it won't work, just that you will have to "figure it all out". Some manufacturers do provide assistance in setting up their products with 3CX, you might want to check the website of the manufacturer of any you have in mind. And make sure that there is a favourable return policy.