Assuming you get the full file name ...

dir /b/S *.dsp | perl -e 'BEGIN{@ARGV=map{ chomp and $_ }<ARGV>}' \
                      -i.bak -pe "s#\bGPP\b#Arjun#i;"
[download]

No longer a oneliner, but it should do the job.
HTH

_________
broquaint