This likely differs a lot between countries (though there should be a common core due to ITU of what is being regulated how by whom); in my country the cellular frequencies were auctioned off for dozens of billions of euros to the cellular service providers, with strings attached, e.g. that the networks had to at least use and support 3G.
The same regulatory body (BNetzA) is also responsible for licensing telecommunications providers in general. This is probably the crunchpoint - if you're not a licensed telecommunication provider, it's simply illegal to offer telecommunication services.
While this is mostly true, in practice (at least on the 450-470 MHz band I work with), you can give fairly vague purposes, like "telemetry" or "SCADA", and it doesn't matter what vendor or what protocol you use. In fact, there is no section of the licence that says "I am going to use protocol X on equipment Y manufactured by vendor Z" or any part thereof (emissions designators are a different story).
I don't have any experience will cellular licences, though. Given that carriers frequently repurpose spectrum as technology changes, I can't imagine their licences say "GSM only" or anything like that. The band that used to be 3G GSM is being used for 4G LTE on my phone right now.