Up front I'll warn you I don't use gimp, but the symptoms you describe suggest that the Windows icon is set up to switch to the directory where gimp.exe is located before running the program and it finds the 'missing' .dll because it is then in the 'current directory'.

A possible fix is to arrange for your program to switch to that directory before attempting to start the program. One way of doing this is to combine the commands required into one something like this.

system 'c: & cd \Program Files\GIMP\bin & gimp.exe';
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