Help with using Gmail attachment and custom GPT to output file

Hi, I have tried for a few hours, watched many videos and read a lot on here, but I am unable to get this working.

We have a customer GPT already created that does what we want, at present we upload the .pdf file into Chat CPT custom GPT within Open AI website, and it display the data we want in the chat below it with a save button to download the .csv version. It’s basically taking product date from a .pdf file and turning it into a .csv file.

We want to automate this with make.com, so that when an email with a PDF lands in a Gmail folder, the customer GPT runs automatically and outputs the content to a .csv file in a Google Drive folder.

The scenario we have built is as below:

(Gmail) Watch Emails > (Gmail) Iterate Attachment > (openAI) Upload a file) > (openAI) Create a completion > (Google Drive) Create a File From Text.

This is kind of working, except the file being output into the Google Drive folder is dummy data, as nothing in this scenario (that we can find) allows us to link the openAI module within the scenario to the customer GPT we have created to actually run and generate the output we want.

Feels like it should be something more like the below:

(Gmail) Watch Emails > (Gmail) Iterate Attachment > (openAI) Upload a file) > (openAI) run customer GPT > (Google Drive) Create a File From Customer GPT output.

Any help would be greatly apprechiated, thanks.