Welcome to LQ.
'make clean' will delete all *.o and ns* binary. Except indep-utils/* /*.o.
Saving the original 'ns' :
cd ns-allinone-2.<?>/ns-2.<?>/ && strip ns && cp ns ns-orig ;
su ; # make install ; # cp ns-orig /usr/local/bin/
( 'make install' will copy 'ns' to /usr/local/bin/ ).
When the "ns GPSR" has been created : strip ns && cp ns ns-gpsr ;
su ; # cp ns-gpsr /usr/local/bin/ .
... Then you have both versions in your PATH.