Can I use my Gmail account as Apple ID?

Gmail addresses can be used (I use one) for your Apple ID. The following from Change your Apple ID - Apple Support may help:

If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it. 

Why would you want to change your Apple ID from your iPad if you had already done it from your PC? You only change your Apple ID once to a new email.

The following for what you may need to do after changing it may help: What to do after you change your Apple ID or password - Apple Support