Why can’t I use my gmail account for an Apple ID

What happens when you try? Error message(s)? If you are seeing that the address isn't available, 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 Reachable at or Rescue e-mail address, then it can't be reused with another Apple ID 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 to use as your Apple ID or you can try waiting 30 days to see if that works.

There have been posts by people who have contacted Apple Support and they were told that if you delete an ID associated with an email address, you can never reuse the email address for an Apple ID.  You will need to get another e-mail address to use as your Apple ID.

You can use any email address for a new Apple ID. You can also change an existing Apple ID from one third-party address to another, like from @hotmail.com to @gmail.com. However, if your existing Apple ID is an @icloud.com, @mac.com or @me.com address, you can’t change it to a third-party one.

See the instructions below for changing an Apple ID. Note that you can use gmail depending on your circumstances.

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 documentunder 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.”

There may be some redundancy, but see What to do after you change your Apple ID or password 

 https://support.apple.com/en-us/HT204071

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If you want to create a new Apple ID, then see How to create a new Apple ID https://support.apple.com/en-us/HT204316

Some of these things will apply to your situation even if you are not changing your Apple ID: What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

 Keep in mind that some things available under a first Apple ID will not be available under the new Apple ID. For example, you won’t be able to transfer anything purchased from the App and iTunes Stores to the new ID. And, there are other issues to be addressed. See

what Eric Root say on this matter here:  https://discussions.apple.com/thread/251263630

what Winston Churchill says about it here: https://discussions.apple.com/thread/7452747