If you have set your Go Pro to capture a sequence of images then here are some easy steps to convert them into a movie with ffmpeg.
My GoPro Her0 3 Black numbers the videos in the following format:
Where the first 1 represents the batch of photos, e.g. if I took 3 timelapses on the same SD card before formatting the sequences would look like:
To use with ffmpeg you need to use the image input filter and unless your image happens to start at 1 (almost never happens to me) you also need to specify the -start_number, especially if you decide to manually delete a few images at the start as I sometimes do.
If the first image was G0010582.JPG you would use the following command:
ffmpeg -f image2 -start_number 002 -i G0010%03d.JPG -vcodec libx264 -b:v 5000k -s 1920×1080 timelapse_1080P.mp4
Note that with this setting I am outputting a 5Mbit video, you can drop this all together if you want to get a lot higher quality or manually up it to 5-8Mbit for decent quality HD video while maintaining a reasonable size.
I also find ffplay is quite good for a quick view on how it looks: