If you are using the bash shell (on Linux/OS X/using Cygwin) is to add you ~/.bashrc file:
gvim () { command gvim --remote-silent "$@" || command gvim "$@"; }
On Windows I think you could have a gvim.bat batch-script to achieve the same..
gvim.exe -p --remote-tab-silent %1 %*
If gvim.exe isn’t in your path
Run > Search "Environment"
Edit PATH var for current user or system.