When you say you’ve tried overriding this text in functions.php, have you been following the information provided in the gform_validation_message docs? You should be able to change the message for all forms, or to target a specific form in order to override its default validation message.
This will be the best way to modify that output. Generally, it’s not recommended that you change the plugin files directly, as those changes will get lost next time the plugin gets updated.
Hopefully that helps. If you’re still stuck, feel free to share a snippet of the code you’re using and we can help you troubleshoot.