Hi Vinay,
You can implement the Custom Head HTML as the following to a Marketo Landing Page -
Go to Landing page editor, and drop down options from "Landing Page Options" - Select "Edit Meta Tags". Then, you can add the GA script for the Head HTML.
![0EM50000000STMg.jpg 0EM50000000STMg.jpg](/t5/image/serverpage/image-id/5618iC7A58AA0DA0EBCE5/image-size/large?v=v2&px=999)
Hope that helps!