Why?

The make installed by default ( /usr/ccs/make ) is not really used for building software. The one that comes with SunStudio (dmake) is the one that should be used.

The only real reason for using gmake is to compile code which uses gnu makefile which do not always follow proper makefile rules. Often makefiles that require gnu make also require the entire gnu build chain, and these do not comply with standards, where Sun versions do.