I don't think it's going to be as clear as recommending specific skills to you. ‘Gameplay’ engineer is a catch-all term for anyone that isn't really specialising (e.g. in rendering, networking, AI, tools, etc). So, you just need to be making games and ensuring you have a wide knowledge of the general process. Can you read input and have a player character respond to it? Can you implement in-world gameplay systems such as jumping, shooting, zones that trigger effects, spawning, etc etc… and can you implement more abstract systems like timed effects, skill trees, experience points? How are you with UI - both overlays and more complex menus or screens? Audio? NPC AI such as navigation + pathfinding, behavior trees, or utility systems? Save/load systems or checkpoints? And can you do all this with an eye on performance? And while keeping the code clean and maintainable?

If there's just one thing I would say you would need for a gameplay engineer that you might need a bit less for other specialties in gamedev, it would be the ability to interface well with designers. That isn't just things like knowing how to read a design doc or interpersonal skills in general - all programmers need those! - but things like:

• thinking about the interfaces that the features you build present to content creators
• asking questions to resolve terminology ambiguities in the docs
• building a spec from vague wishes on the part of design/management
• spotting cases when a designer has misconceptions regarding how your features work
• determining what a designer is trying to accomplish with a feature request, rather than what they think they need to do
• being familiar enough with design goals to be able to suggest alternatives when a feature request cannot be fulfilled

I don't think you can get any of these skills through anything other than experience on a team where the designers are non-programmers. So, my advice to cultivate these skills would be to find an artist/designer and do game jams with them.