Since version 76 of Google Chrome (July 30, 2019), you aren't asked for permission to enable Flash anymore. However, to make use of Gynzy, Flash needs to be enabled.
In order to enable flash in Google Chrome for Gynzy, you can click the lock icon 🔒 on the left hand side of the address/URL bar. Then, next to "Flash" choose Allow.
If you or your school have Flash already enabled, and Gynzy is still not working. You can click the link in the bottom of the notification screen to try to enable Flash from within Gynzy.