May I suggest the batch mode of Irfanview, which only runs on Windows. Irfanview is also good for simple edits, but it is no GIMP.
Once installed, select "File | Batch Conversion/Rename". From there it gets a bit tricky, you need to select all the files, you need to specify a destination folder, and you need to click on the Advanced Button to see Advanced options. It can rotate +/-90°, but to do 180° you will need to pick fine rotation on the far right and enter in 180°. Note that you can also resize them, change JPEG quality, and rename them.
You can also do it rather quickly by staying in the GUI and using the keyboard. Open any file in the folder of pictures. Press 'R' two times to rotate twice. Then press Ctrl-S to save. Two windows will show up, one your typical file dialog, and a second one for JPEG compression (if GoPro's default format is JPEG). I would make sure the Quality Factor is at least 90%. Then save the file; you should be able to press the Tab key or Alt-S to save. An "Are you sure" type dialog might show up (might be able to turn that off). Then, just press the Space bar to advance to the next file in the folder. If you have 50+ files, the batch might be quicker. Ten files, I just do the above.
Key thing to Irfanview is that its like Blender, it doesn't ask you if you want Save if you modify an image.