Currently I am manually adding GA tracking to every landing page, but is there a way to add the code throughout Marketo so this tracking is automatically set up?
Outside of Landing Page Templates? No, you have to include the code in your template(s) in some way. Certainly not on every individual page, though!
Most flexible move is to include the embed code itself as a top-level {{my.token}}, then use a mktoBoolean variable (defaulting to true) to control whether it loads, in case you need to turn it off ad hoc.