Freemake will do the trick for 3GP files
I had the same issue, but with 4-5 files not 25. Freemake Video converter will do the trick for free to combine your 3GP: Freemake ? Freeware For Everybody | Free Video Converter, Free Video Downloader
Simply select the files and click join after you click on the format you want it to output.
Even on my high-end Sony HD SD Card camcorder, I have the same problem. You would think that the ability to merge to video files would be an option on the device itself as it seems so basic. As if anyone would want their footage of their kids soccer game to be in 17 different files! My frustration has been finding programs that simply merge the files without having to do 30 minutes of recoding. Freemake will do it in a few minutes.
By the way, if you are looking for a program to combine HD files (.mts2 files) tsMuxer is a great program that performs this function.
Hope this helps.