I tried to change my email address from a yahoo account which I own to a gmail account which I also own and it keeps telling me the gmail address is not available. How do I change it?

Have you logged into your Apple ID to see if the address shows under Reachable At, as a Rescue email address, or as a Notification email address? If it is listed, you can remove/replace the email address.

Apple ID - Manage  

Apple hasn’t published any guidance for using an address again. Normally, if your e-mail address was associated with an ID in any way, for example as a Notification email address or a Rescue e-mail address, then it can't be reused as an  Apple ID email address for approximately one year. I have also seen posts that stated Apple told them there was a 30 day waiting period before the address could be used again. You will need to get another e-mail address from somewhere like AOL or Yahoo to use as your Apple ID or you can try waiting 30 days to see if that works.

and it keeps telling me the gmail address is not available.

Check here -> How to recover your Google Account or Gmail

https://support.google.com/accounts/answer/7682439?hl=en