I’m asking for their email in return they are getting a free gift. How can I be sure they give a good address?
Is there something I need to ‘check’ in the survey or thank you page to be sure it’s a good address?
There is not really a mechanism for making sure an email is a good email.
I’m not sure how exactly you are presenting the free gift, but perhaps you can gate access to the free gift behind an email message.
For example, your respondent will provide an email address and then you can trigger an email to that email address via a Send Email Action:
The email itself can contain information about the free gift.
Hope this is helpful!