If you are interested in migrating from GMX to GMAIL here is a video explaining the process
Sign in to your Gmail Account and once you are inside your inbox click on the gear icon located on the top right corner, then you have to click on see all settings followed by the accounts and import tab and click on import mail and contacts.
A pop up window will appear in which it will ask you to enter your email account and password. Select what information you will like to import and close this window as the import process will begin. You will receive a confirmation notification on your inbox when its done. Locating your imported emails is easy, just search on the folders on the left corner under your GMX email.