I think is not possible using gwidgets
. You can set andle a "resize event or signal".
One alternative is to use RGtk2
(GTK+ 2.0):
library(RGtk2)
win <- gtkWindow("toplevel", show = FALSE)
gtkWidgetSetSizeRequest(win, 500, 500)
win$show()
gtkWindowSetResizable(win, FALSE)