Is my phone GSM or Global

That would not be the model number of your iPhone 8. Instead, it would be Order Number. The "ZP/A" part of that number indicates that it is designed for the SouthEast Asia region. However, when I entered the full number you provided, it was not found.

Do you also have the actual model number? An iPhone 8 would be something like: A18XX or A19XX

Well, that's where Apple states that you can find it. Again the number you provided does not help identify the actual model of your phone to know if it is a GSM or Global. Sorry! As such I suggest that you take it to your local Apple Store or Apple Authorized Service Provider and they should be able to answer your question.

No, that's not it either.

Ref: Find the model number of your iPhone, iPad, or iPod touch - Apple Support

Never share your imei number!,it is as if you shared your credit card number.