It's easy to get hung up on the word "strategy." There's no magic in it: a GTM strategy is just a documented set of decisions and a plan.
It's words on paper, and the purpose of it is nothing more than alignment. To hit your sales, revenue, and margin targets, you've gotta get and keep everyone on your team aligned on what needs to be done, why, by whom, and when. A good GTM strategy document answers all the questions mentioned in other answers. Questions about buyers & pain points, budgets, product overview, competition, differentiation, pricing, partners, sales & marketing plans, success criteria, etc.
A great GTM strategy, however, will inspire your whole team to row hard in the same direction and make some magic together. The way you engage your team (or don't) in the process of creating your GTM strategy, and the way you communicate it, makes all the difference. Give me a call if you'd like some help getting that right.