For countries where the G Pay app is not available but cards can be added to use G Pay (from Revolut, Curve etc...).
There's no easy place or action where you can check your existing cards or use a different one other than the default. You can do these only going deep into the settings and it can be bothering.
After installation a quick tile action becomes available to choose and put into the quick actions (at the top where other quick action buttons are located like wifi, bluetooth, etc..).
When you click on this action button, the G Pay card settings page will open where you can see your cards and pay with any of your choosing or add new cards / remove old ones.
The quick action tile will show as active/inactive according to your phone NFC status.