is actually a part of vim
distribution. It's really a superset
of vim - it provides all the vim functionality and then some. gvim stands for G
Some of the additional features include:
Because gvim runs as in X11
or in Windows
it provides native methods of clipboard use. That is you can select stuff in X and expect it to be copied on middle mouse button click
in other window. Basically gvim is just a prettier version of text mode vim.