m2ts to 3g2 conversion represents transformation of MPEG-2 video stream (Blu-Ray for example) to 3GP format, represented by the .3g2 file.
You can use any video converter listed bellow or use popular FFmpeg encoder a command line tool. Install FFmpeg to your system ► Open command line (Bash, Terminal) and type:
ffmpeg -i /path/to/your/file.m2ts -vcodec copy -acodec copy /path/to/your/newfile.3g2