I have an entry-level gaming laptop with an NVIDIA GeForce GPU and I like to cap the few games that I play on it to 60 fps. Would an expensive gaming monitor that can handle refresh rates well beyond 60 Hz be (1) a waste of money and (2) problematic? I've read that using an overpowered monitor on a relatively underpowered GPU may cause performance issues. However, can't I just buy a G-SYNC capable monitor and have the NVIDIA GPU bring the monitor's refresh rate down to 60 Hz when I play games?
I think you've misunderstood what people mean. The refresh rate of your monitor has no impact on gaming performance. It only decides how often your monitor can show a new frame every second. What can impact performance is monitor resolution. However you can always choose to render a game at a lower resolution and let it upscale. Or use Nvidia DLSS if availible.
You also do not need a G-sync capable monitor for this, especially considering that with a G-sync monitor the monitor refresh rate is dynamically ajusted to the framerate of your game. Basically already dynamically doing what you are asking. If you then limit the maximum refresh rate. you're only denying yourself extra visible frames if they are availible.
However if you really wish to reduce the refresh rate of your monitor for other reasons you can do this by right clicking on the desktop and selecting display, then scroll down and click 'advanced display settings' there is a drop down to edit the refresh rate.