iTunes doesn't support 3GP as an input format. You may need to convert 3GP files to MP4.
From an article on how to convert 3GP to MP4, I learn that:
3GP (3GPP file format) based on MPEG-4 Part 12, is the simplified version of MP4, especially designed for mobile phone to record and transfer videos. When 3GP format is invented, the mobile phones are less powerful as today, so that the 3GP video is usually in low resolution, low frame rate, low quality and low bitrate. Besides, 3GP file uses AVC/H.264, MPEG-4, and H.263 for video compression and AAC, AMR for audio compression. Since many players don't support AMR audio codec, you cannot hear any sound when playing 3GP files on these players.
Compared to 3GP, MP4 (MPEG-4) is the most widely used video format today with high audio and video quality, and is supported by nearly any popular player and device, like iMovie, VLC, iPhone, iPad, etc.
All in all, it's the wisest choice to convert 3GP to MP4 for playback on most popular players and devices. The following article will introduce a step by step guide on how to convert 3GP to MP4 with ease. Furthermore, 3G2 (3GPP2 file format) for 3G CDMA2000 multimedia services, is very similar to the 3GP file format, so this article also applies to 3G2 format.