GPT was the first in the GPT series, followed by GPT-2, GPT-3, and more recent versions as of my last update.1
References
1Silawat A, Soreng S, Dwivedi G, Kohli HK. ChatGPT: A blessing or a curse in disguise. In: Katrandjiev H, Tanushev C, Bankova Y, et al., eds. Marketing and Strategic Planning in Digital World. Publishing Complex – UNWE; 2024:124-141.