OpenBSD includes gcc in the base install. However, it includes gcc-4.2.1. If you prefer to use gcc-8.3.0, the names of the binaries differ so as to not conflict with gcc in base.

The exception being gnat.

GNUstep is available if you would like to develop using Objective-C. The following should be added to ~/.kshrc:

export GNUSTEP_MAKEFILES=/usr/local/share/GNUstep/Makefiles
. /usr/local/share/GNUstep/Makefiles/GNUstep.sh

You will also need to call gmake rather than make when building Objective-C code. See this StackOverflow post for more information.