LosslessCut is great, but I needed something quick when I cleaned up old GoPro videos. Just open an mp4/mov in QuickTime Player, trim it and save under a new name. It will create a lossless cut. If you need several from same video, you would need to repeat the process on the original file.

(unfortunately, the trimmed video won't get the same metadata, so I also needed to use exiftool -TagsFromFile oldvid.mp4 -all:all newvid.mp4 )