Ok I found the solution. First, it looks like it is necessary to install
libsecret-tools because there is no CLI command to access the gnome keyring using
gnome-keyring (is that true? looks strange to me).
Then you can setup a password with:
secret-tool store --label=Unlocker type unlock-keyring
And call that password with:
secret-tool lookup type unlock-keyring
You will be prompted then, to enter the keyrings password and it will be unlocked.
Of course, you don't need to setup an extra "Unlock" password, thats a bit nonsense because its just used to force the keyring to unlock. You can also choose any of your existing passwords to unlock the keyring, but I liked it more this way.
For more infos about using
secret-tools you can lookup this question as well: Is it possible to use secret-tool to lookup existing seahorse passwords?
If there is another solution without installing an extra
secret-tool, I would like to hear that!