In a word, yes. You'll notice it particularly when recording with the high-end video modes that keep the camera's internal processor working hard as well as transferring large amounts of data to the memory card. The camera and memory card can get quite hot to the touch, even to the point that it can be uncomfortable to hold.
It's especially noticeable with some of the newer designs for the HERO6 and HERO5 lines when you're using the to-end video modes that require lots of processing power and large amounts of data being written to the memory card very quickly. The metal rim around the lens port acts as a heat sink, drawing the heat out to dissipate it. The result is that it can become very hot. And if you take the microSD card out right away after recording in the high-end modes, you'll find that that's hot too.
But you're definitely not the first to ask. Indeed, GoPro anticipated the question and addresses it directly in the HERO5 Black and HERO6 Black instruction manuals. They also offer some good suggestions for mitigating the heat problem and working around it.
And the good news is that there's a safety mechanism built in to newer models--the camera will shut down if it gets too hot and show you a message explaining why it's shutting down (and an icon of a thermometer). While that's frustrating if you're in the middle of capturing something important, it's at least better than causing damage to the camera.
Here's what the HERO6 Black instruction manual says on the issue:
When capturing video at high resolutions or high frame rates in warm ambient temperatures, the camera may become warm and use more power.
In addition, lack of airflow around the camera and using the camera with the GoPro app further increase camera temperature and power consumption, and decrease camera recording time.
If the camera gets too hot, a message appears on the screen indicating that the camera is shutting down. For details, see Important Messages (page 85).
When recording in high-performance video modes, capture shorter video clips when in stationary use, and/or limit your use of features that increase power consumption, such as the GoPro app. To control your
camera remotely at higher temperatures, use a GoPro remote or Remo (Waterproof Voice Activated Remote) (both sold separately) instead of the GoPro app.
If it does get to hot and shows the CAMERA TOO HOT warning, there's only one thing you can do--let it sit to cool down for a bit before trying again. How long that takes is going to depend on the conditions.
One other consideration is if you're using the camera in a dive housing. It's pretty common for the warmth of the camera to cause fogging inside the housing when you take it in the cooler water. That's where anti-fog inserts come in handy. You don't need them--and they won't work--with the built-in housing of the HERO5 Black. It's only if adding another layer of housing such as the Super Suit dive housing for the HERO5/6 Black or a standard housing for the HERO4 Black.