How to change the default license selection in `civix generate:module`?
I believe you can use any of the licenses in https://github.com/totten/license-data/blob/master/index.csv, which is used by civix.

In your case, just add --license=GPL-3.0 to the command.

It looks like you may be able to set a value in ~/.civix/civix.ini to provide a local default: https://github.com/totten/civix/blob/v16.03.2/src/CRM/CivixBundle/Command/InitCommand.php#L118