Definition
GPT (Generative Pre-trained Transformer) is OpenAI's family of large language models, and the models behind ChatGPT. The name describes the method: a transformer network is pre-trained on a large body of text to predict the next token, then tuned to follow instructions and generate new text.
The lineage runs from GPT-1 (2018, 117 million parameters) through GPT-2 (2019, 1.5 billion), GPT-3 (2020, 175 billion, which introduced few-shot prompting and OpenAI's API business), GPT-3.5 (the model behind ChatGPT's launch in November 2022), GPT-4 (March 2023) and GPT-4o (May 2024). OpenAI then ran a separate o-series of reasoning models before folding reasoning back into the main line with GPT-5 in August 2025. Point releases followed through 2026, including GPT-5.4 in March, GPT-5.5 in April and the GPT-5.6 family in July.
The current generation is GPT-6, released in three tiers:
- GPT-6 Astra (September 3, 2026): the most capable and most expensive tier, aimed at long, end-to-end work such as coding, research and computer use. Knowledge cutoff April 30, 2026.
- GPT-6 Sol (September 22, 2026, with GPT-6.1 Sol on September 29): the general-purpose tier for complex coding and agentic workflows. Knowledge cutoff April 20, 2026.
- GPT-6 Luna (September 22, 2026): the low-cost tier for high-volume tasks, priced at $0.10 per million input tokens. Knowledge cutoff May 18, 2026.
All three are reasoning models with adjustable effort levels, accept text and images, and offer a context window of about 1.05 million tokens, according to OpenAI's model documentation. On October 7, 2026 OpenAI brought GPT-6 to everyone in ChatGPT, with Sol serving paid plans and Luna serving Free and Go users, and said ChatGPT had more than 1.2 billion weekly users. The same launch added Intelligent UI, which lets answers include interactive charts, maps and tools.
GPT models reach users through two channels that matter for GEO. The first is training: what the model learned before its knowledge cutoff, gathered partly by OpenAI's GPTBot crawler. The second is live retrieval, when ChatGPT searches the web through OAI-SearchBot and cites the pages it reads. OpenAI says GPT-6 makes better decisions about when to search and starts answering web-search questions 44% sooner than GPT-5.6 Instant. Recent cutoffs (April and May 2026) mean the models know more of the recent web without searching, but anything newer, and any page OpenAI's crawlers cannot reach, is visible only through retrieval.
GPT models also power thousands of third-party products through the OpenAI API. Each integration may or may not enable web search, so the same brand can be described from training data in one product and from fresh search results in another. Practical GEO work covers both: keep authoritative, consistent facts about your brand published widely enough to be learned, keep key pages crawlable for OAI-SearchBot, and track ChatGPT answers after each model change, since a new GPT version can shift which sources get cited.
Examples of GPT (Generative Pre-trained Transformer)
- After ChatGPT switched its paid chat model from GPT-5.6 Sol to GPT-6 Sol on October 7, 2026, a SaaS brand reruns its tracked prompts and sees its pricing page cited in 9 of 40 answers instead of 4.
- A support platform routes ticket classification to GPT-6 Luna at $0.10 per million input tokens and escalates only multi-step refund cases to GPT-6 Sol.
- A product launched in June 2026 is unknown to GPT-6 Sol without search, because its knowledge cutoff is April 20, 2026; the company focuses on getting its launch pages crawled by OAI-SearchBot.
- An engineering team migrating off the retired o3-deep-research API model in July 2026 moves its research pipeline to GPT-5.6 Sol, as OpenAI's deprecation notice recommends.
