What do you mean when you in my Paypal account is asked for a Secret? and where is GBP?

Paypal Checkout Registration ask for my Client ID (is that my e mail adress?) and then for a Client Secret. I have no idea what that means.

You can get the Client ID and Client Secret in the link: https://developer.paypal.com/

Here is the complete guide about PayPal Checkout integration and getting the Client ID and Client Secret: https://www.jotform.com/help/542-How-to-integrate-PayPal-Checkout-with-your-form