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Preview will do it, but it does a very poor job of converting the image to 256 colors (required for the GIF format). I'm not sure what the problem is with Graphic Converter...
 
iPhoto can export to different formats (File==>Export or Share==>Export in older versions) and can alter the resolution, Preview can export to GIF.

Both are included with your Mac ;).

Otherwise you could try Seashore.

I am confused as to why you want to convert, JPEG is generally a better format.
 
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