I am getting volumes of spam mail on my IPhone Gmail but it doesn't show in my desktop.

Are you using the Apple Mail app or the Gmail app on your iPhone to access your Gmail?

SPAM is an issue for all of us. Preventing it getting into your iPhone's "mailbox" will continue to be a challenge, but they're things you can do to try to reduce it. Changing your phone number alone, will not work. It may stop incoming calls from "bad actors," but once your new phone number gets found, they will be back.

Same issues with iMessages, SMS text messages, and email.

Staying with emails, it is best to attempt to stop them at the "source." In this case, your email provider's mail servers. Gmail, like others, offer methods to configure your email account to reduce the number of spam messages from getting to your iPhone in the first place. That is where I would start. Once you exhausted that source, the next step is to try to do the same with your iPhone. That is where we would need to know what app you are using to access your email with.

Note that the support article you arrived here from specifically targets your Apple ID's iCloud email account. It will not help you with your Gmail. Alternatively, you have two options, as well: 1) Use Gmail's spam filters as the first line of defense, and then, forward your messages to another email account to do the same, or 2) Stop using Gmail all together.