Who are all the stakeholders that can provide input to a product roadmap? What are the stage gates to help decide if something should land in the roadmap?

I like to get as much input as is reasonable both internally and externally.

• Internal examples of stakeholder input: partnerships, sales, marketing, product support, operations, finance, data science, design, execs… 

• External sources of roadmap input: press reactions & professional product reviews, app store ratings and reviews, customers support requests, user research, inbound marketing research, and above all behavioral data from your logging/analytics.

In terms of gates... during the planning process, you don't really have gates, just the process you follow that encourages all good ideas to flow in, but then rates those ideas by expected impact toward your goals, and level of effort to design/build/test/release. See my other answer to the question about the end-to-end roadmap prioritization process.

When the team is executing, not planning, ideally all ideas coming in can wait for your next update or refresh cycle. This might be quarterly or even monthly in a very lightweight way... but since your teams have established goals, new projects will only bump other projects out of the roadmap if they're more likely to have higher impact in achieving the goals.

Occasionally there's a disruptive change that should cause you to think through changing the team's goals or an immediate pause/change to work-in-progress in the roadmap. But these should be rare, and very important, because you're going to thrash your team to consider the change/pivot.