Use Google's Transfer your Content option to transfer all files you own and/or emails to another Gmail account.
Click your picture on the upper right from your account and then choose Manage your Google Account:

Under Home or Data & Privacy you should click Start Transfer from the Transfer your Content option:

You will need to provide a Gmail account it is being transferred to. From the destination account you will be sent a verification code you will need to provide here:

Now you have the option of choosing to transfer files you own and/or your emails:

Once completed an email will be sent to the destination account with a link to access the files.