A GPT in ChatGPT isn’t really “trained”, it is just informed. It can perform a search on the text within uploaded documents (but diagrams or images within remain elusive).
That gives you the opportunity to round up all the concrete knowledge the AI might need to have on hand to help you out, after you give it an attitude of being an Arduino Expert in instructions. Product pinouts, ATmega chip datasheets, programming manuals. Upload to GPT you create in ChatGPT plus and test the waters. The AI itself will already have knowledge.
Here’s a chat share just talking to ChatGPT about some random idea: