gTLD stands for generic top-level domain. It's a category of domain extensions maintained by the Internet Assigned Numbers Authority (IANA) to classify the Internet, and for the most part, how gTLDs are registered and managed is consistent, regardless of which specific gTLD you have. This is because gTLDs are under the control of the Internet Corporation for Assigned Names and Numbers (ICANN), the internet's governing body.
gTLDs are different from ccTLDs (two-letter country code top-level domains). gTLDs are made up of generic terms, like .COM, .ORG, or .PHOTOS. that don't have a specific geographic designation. For ccTLDs, each country registry can set its own policies, processes, and prices, so they can be very different from gTLDs and from other ccTLDs.
Historically, the most common gTLDs have been .COM, .INFO, .NET and .ORG, but ICANN has delegated hundreds of new gTLDs for professions, hobbies, events, and more.
These new extensions are often called nTLDs, or new top-level domains. While not a true classification, most will identify nTLDs as any of the hundreds of new gTLDs that started being released in 2014.
If you're interested in reading more about gTLDs, here are some handy links: