In a company in which GTM, product launches, and roadmaps have until now been handled by C-level execs, how should product marketing "break in" to try to influence and get involved?

First, check in to ask what the roles and responsibilities have been up to now. If they haven’t been documented, do a mini “listening tour” with the key players to offer to document how things have gone and what they think could be better. Listen closely for what they really care about. Ask to get your hands on other core documentation that has held together GTM plans. 

After reviewing what has been done, think about ways you can demonstrate how you can drive efficiency and clarity in the overall plans and execution process.

Understand what aspects of GTM, launches, and roadmap planning execs feel most invested in having a stake in. Clearly share a proposal for review checkpoints and explain how you’ll involve them in early strategic mapping to make sure they are able to say their piece. 

If they are still reluctant to give up the reins, ask if you can help take discrete parts of the workflow to build up some trust with them. What have they struggled with? What is time consuming for them? Where do they perhaps have knowledge gaps that you can fill?

Lastly, make sure you understand what their primary goals are. You need to diagnose why they feel the need to keep this important work to themselves. Are they worried about hitting revenue targets? Are they concerned that people at other levels won’t be able to drive decision making fast enough? Are they especially attached to a certain part of the strategy work that they are passionate about? Knowing the core reason for holding this work tightly can help you build their trust in the right areas so that you can take work off their plate and demonstrate your own ability to lead this cross-functional work!