What advise would you give a junior PMM who has minimal GTM experience interviewing for PMM roles?

If you don't have the opportunity to manage an entire new product launch, you'll have to get creative. See if there is an opportunity to own a specific piece of a launch someone else is managing or even manage a smaller feature launch. I'd also look for opportunities work with the product team, even if it's not on a specific launch. Big bonus points if you work with the product team on research or roadmap prioritization elements. 

Let's say none of those opportunities are available - you can still focus your time on cultivating the skills that make a great product launch PMM. Beyond the tactics, the skills and experience I look for when hiring product launch PMMs include the following and I think you can work on these in a variety of different projects:

• Someone that really knows how to work with product teams and understands how they work
• Can manage a broad range of stakeholders
• Strong and bold storyteller 
• Project management skills

The biggest thing for newer PMMs is to learn and understand what strong PMM looks like, even if you haven't experienced it yet. Meaning, being able to answer an interview question with: "While I haven't had experience in this particular area, the way I might approach this situation in the future is... x, y, z." 

In addition for interviewing, not only articulating what you've worked on specifically but how it fit into the larger PMM strategy (even if you didn't contribute to creating the plan, that's okay). When I help with interviewing, I'm much more interested in whether the person in front of me has that type of growth mindset. It's really easy to teach PMM skills; it's really hard to teach someone how to learn on their own.