Download the model, and go to C:\Valve\Steam\SteamApps\*your email*\half-life\esf\models\player. Then, if your .mdl file has a name other than one of the existing esf characters, name it goku.mdl (unless otherwise stated in a readme or something), drag the original goku.mdl file from the goku folder and put him in the vegeta folder (cause theyre pals and youll want a backup). Now put the gogeta model (named goku.mdl in the goku folder. Chances are the model comes with instructions though.
Only you will see the model, and everyone elses Goku will look like Gogeta, but oh well.