You will find several Maserati registries available online. They allow for research and verification of the information on your (future) classic Maserati GT's.
This list is an overview of the publicly available and actively maintained registries on the internet. If you want to add info to this list, please contact us at info@classichemasters.com.

3500 GT and GTI Touring: http://www.am101.org/

3500 Spyder: There is no public and actively maintained register available.

Mistral Coupé: https://join.classichemasters.com/c/mistral-registry/

Mistral Spyder: https://join.classichemasters.com/c/Mistral-Spyder-Registry/

Sebring series 1 and 2: https://join.classichemasters.com/c/sebring-registry/

Mexico: http://www.mexico-registry.com/

Ghibli: https://www.ghibli-registry.com/

Ghibli Spyder: https://www.ghibli-registry.com/

Indy: https://join.classichemasters.com/c/Indy-Registry/

Khamsin: http://maseratikhamsinregistry.net/

Bora: https://join.classichemasters.com/c/Bora-Registry/

Merak: https://www.merak-registry.com/

Quattroporte: http://www.tipo107.com/

Quattroporte 2: https://join.classichemasters.com/c/Quattroporte-2-Registry/

Quattroporte 3: https://join.classichemasters.com/c/Quattroporte-3-Registry/

Kyalami: https://join.classichemasters.com/c/Kyalamin-Registry/

5000 GT: Work is in progress on this register. Website will be available at maserati5000gtresearch.com.

The register of Maseratis that French importer Thepenier sold over the years: http://www.thepenier-maserati-register.com/

To uphold the value of these classic Maserati GT's, precise documentation of the provenance and ownership history of each car is essential.

In its most basic form, a registry is a list of cars by model, year, and owner. A beneficial registry also contains records of serial numbers, colours, options, modifications, ownership history, photographs and data or photos from car shows, events, magazines and online publications as well as auction or sale results.

As a worthy bonus, a registry may also contain information from the factory archives. Maserati Classiche in Modena, Italy has a wide range of documents available containing information such as manufacture date, original colours and options, delivery date and destination, original buyer, and sometimes even original build sheets.

Disclaimer
Registers are an excellent starting point and a superb source of data for researching classic Maserati GT cars. They are the result of the efforts of volunteer enthusiasts. Keep in mind that registers are by nature "works in progress" and may be incomplete or contain inaccurate data.