Ok, the code you have written MUST be GPLv3 licensed and you can charge, however anybody receiving the code could if they wanted make it public and free for anyone if they so desired. Start here: https://www.gnu.org/licenses/gpl-faq.en.html#DoesTheGPLRequireAvailabilityToPublic and https://www.gnu.org/licenses/gpl-faq.en.html#GPLCommercially.
General help
Moodle GPL license
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