As of now, gerbv uses the default GTK theme on MS Windows, which looks quite alien to most Windows users.
Fortunately, there is a way to make GTK use the native Windows theming API, which means that the application will look like any other Windows application: it will use native buttons, menus, font sizes etc.
The way to do this is to use the "wimp" rendering engine instead of the default. AFAIK it's integrated into GTK.
I attached an example gtkrc (coming from the Camelbox project) that shows this plus some other settings that make the application look even more Windows-y.