If you are using an Android app:

In that case, you will need to open the Transactions section and check at the bottom of the screen. Your ID will be shown there.

If you're using a web version:

Open your profile by clicking on your name in the upper right corner. Your ID will be shown in your profile.

If you're using iOS app:

Open your profile by using the button in the lower right cornet. Then choose Settings (a small gear icon) in the upper right corner. There you can find your GC account ID.