How can I transfer my WhatsApp backup from gmail account

Sorry, but you can only restore a WhatsApp Backup on an iPhone, if it has been saved to iCloud.

A backup made by an Android phone can't be used.

However, sending the Chat via Mail could be a solution:

...

You can also email yourself a chat history if you would like to save a conversation:

1. Open the WhatsApp conversation you want to email.
2. Tap the contact's name or group subject in the navigation bar.
3. Scroll to the bottom and tap Export Chat.
4. Select if you want to Attach Media or email the conversation Without Media.
5. Select the Mail app. (Or tap More for more options.)
6. Enter your email address and tap Send.

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from: WhatsApp FAQ - Restoring your chat history

Android and iOS are two different "languages". No matter which side you are switching to, backups from each side can't be used on the other.