How to modify audio level of video according to replaygain info?
FFmpeg, a free command-line tool, has a volume filter that should do this.

Syntax would be

ffmpeg -i input.mp4 -c:v copy -af volume=replaygain=track output.mp4


The af invokes the volume filter with the option set to adjust the volume using the track replaygain value (if present). Video is set to passthrough into the output.