If so, I guess you can make such a "GPT" on your own server and independent from a specific LLM service by using a prompt like
...you have available an API "WEATHER_API". If you need the
weather for a given city for a given day, please say
WEATHER_API('berlin', '2022-11-24')
and I will give you a new prompt including the data you
asked for...
Or is there some magic done by OpenAI which goes beyond this?