Most common problem today is video camera recordings. Many people make use of the GMP4 format for their camera recordings. Playing GMP4 formats are covered on this website here. Now converting the file is another issue and you might want to send the video file to someone who does not have the codec. Converting it first would help.
The converter is the same company which made the codec software. You can download their converter below which has the codec built in already. This is take your video file being in GMP4 and convert it into an AVI file. The link below will take you to the converter which you can download.