Every GTM strategy starts with positioning and messaging.
A detailed and well-aligned positioning document is essential. This should include personas, the problem you're solving, how the solution is defined, what the competition is, the positioning statement, and use cases (it can include more or less depending on the size of the launch and your specific scenario).
From there, the messaging should be defined. And only after this should the GTM channel strategy be created.
At minimum, these three documents must be created. Hope this helps!