GP is the file format - a container format for MPEG-4 video, whereas 3GPP is more a collaboration agreement between companies to make a worldwide standard 3G mobile phone system, and presumably the 3GP file format is the outcome of it's efforts, as is the UMTS standard. 3GP files have quite strict restrictions on resolution, codecs etc. so an encoder that encodes to the 3GP format will produce files that should play on any other 3GP compatible device.
3GPP2 is not to be confused with 3GPP - 3GPP2 is another agreement which sets standards for CDMA2000.
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