Signatures in Google channels are just like signatures in email channels, so this article is lifted verbatim from here.
You can create a separate signature for each group in Zammad. The individual signatures can be created and edited here.
Afterwards, the existing (and active) signatures are available in the group editing mask:
Each group can be assigned its own signature, but they can also all use the same signature.
To individualize the signatures, it is possible to automatically load specific
information into a signature via Variables. All information
stored on the ticket, assigned customers or agents can be inserted.
This makes it possible to design the signature individually.
To load a list of available variables, enter two colons
::) into the Text box of the signature editor.
Please keep in mind that specific information might not be available during ticket creation. The best example here is the ticket number / id. Specific information are created with submitting the ticket and thus are not available before submitting.
Here is an example of a signature with variables and the result when you write a mail: