How do you think about organizing your team (by vertical, persona, etc.)? How do you balance mapping to product versus GTM teams?

The short answer is this is highly dependent and should be greatly influenced by the product and the company that you are part of. This is the dilemma of all things product marketing, there is no one size fits all nor silver bullet answer. I've had equal success with pod models, where a product marketer is embedded with a cross functional teams to work on a particular product or vertical as I have had with creating teams solely focused on bringing products to market. I advocate begining by assessing the individual strengths and professional interests of your team. I'd also work with your partners on product to understand the same and work together to map resources towards goals.

At the end of the day a successful product marketing team is going to be comprised of individuals who are excited and engaged by the work they do and the team they work with.

So first of all, the biggest product marketing team I've built is two (and I was half of it). I'm really much more knowledgeable around establishing PMM within the org. But I'll take a crack at answer the question because I certainly have studied scaling PMM orgs (and been part of a few). 

I think you really need to start with product ownership. If you're going from one to two, make sure that gives your PMMs more time to get deeper product ownership. That usually coincides nicely with how the product management team is being built out. So PMM has one or maybe two direct partners in product and there's alignment between the two orgs. Scale your PMM org from there until product(s) are fully owned and you're at a 1:3 PMM to PM ratio. 

From there, if you're growing PMM one idea I'd consider is sales enablement sometimes known as solutions marketing. A different wing of product marketing but if your selling enterprise (long sales cycle, multiple buying personas, etc) you'll want a team of enablers & learners to be close to sales helping knock down doors and take sales conversation insights back into the marketing and prodcut orgs. Can't understate how important (and often missed by PMM) those insights are. Hire 1-2 solutions marketers, probably align with your sales motion (smb, mid market, enterprise).