i want to use my google account as my apple ID

See the instructions below for changing an Apple ID.

Change your Apple ID  https://support.apple.com/en-us/HT202667. Read these two sections very carefully to see what is allowed: If your Apple ID is a third-party email address and How to change your Apple ID if it ends with @icloud.com, @me.com, or @mac.com.

Before changing your Apple ID using a computer, sign out of all Apple services that use your Apple ID. If you are going to use an iPhone, iPad, or iPod, then stay signed in with your Apple ID on only that device. This instruction is taken from the above document under the heading Before you change your Apple ID.

For sign out purposes, see Where can I use my Apple ID? https://support.apple.com/en-us/HT202659 “This list is provided for informational purposes only and is subject to change without notice. It's not intended to include every feature or service that uses Apple ID. Visit the Apple ID support page for more information.”

And see What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

You can use a gmail email address as the primary email login for your AppleID. See the links Eric posted.

Note the restrictions in the article.

Apple ID Name Change.        

 What to do after you change your Apple ID or password - Apple Support    I suggest doing this before you change your Apple ID or password.