Is it worth re-encoding GoPro footage to H.265?
If your only goal is to save storage space and you never plan on editing or re-encoding the files again, you probably won't notice any change in quality.

However, if you plan to edit or re-encode to another format in the future, I'd strongly recommend against re-encoding to H.265 now. Every time you decode and re-encode video to the same or a different codec, especially highly compressed codecs like H.264 and H.265, you lose quality. With these highly compressed codecs, the generational loss and concatenation artifacts become noticeable to the average viewer in as little as one codec cycle.