Just a bit more clarity is needed.
It is important to understand that files placed in a GPT have file extraction run on them to extract contents, and as you can imagine, Excel to plain text will have many potential problems. Formula without a rendered value, multiple pages, the final product in an unseeable pivot table, etc.
Even more, large file text is broken into pieces, and not guaranteed to all be seen at a time.
Even more, the AI must know to search and what it might find, using a tool it has, writing its own query, and getting back document chunks that have ranked similarity.
Then you can examine how the process worked.
That should let you see that you should come up with your own exporting from Excel, and see that the text presentation of information as sent to the AI as part of a message is clear. You can use actions of a GPT to serve information from an API.