I am unable to use Google pay in fiverr app
I’m afraid they might not be accepting google pay. You can check at their help page what payment systems they are accepting
In most locations, purchases on Fiverr can be made by using one of the following payment methods: Credit Card, PayPal, Fiverr Credits or existing Fiverr Balance.
The above is from the Terms of Service at the bottom of the Fiverr main page.
Everything as to how Fiverr works is explained there in great detail.