Is it possible to register any new gTLD?
No. First of all, there are many TLDs that are closed: no registration services will be offered. Examples are brand TLDs (.bmw), or TLDs intended for own use (several TLDs applied for by Amazon or Google).
The TLDs which will be available for registration might have their own restrictions. For example a local presence for a geographic gTLD, or being group of the community for community TLDs like .dentist or .lawyer.
However, there are many TLDs that will be open to everyone. Here we will publish a list of TLDs and their relevant information, and update it when more information is available.