library(tcltk)
w  <- 100
h  <- 75
tt <- tktoplevel(width=w, height=h)
g <- paste("+", c(round((as.integer(tkwinfo("screenwidth", tt)) - w)/2),
                  round((as.integer(tkwinfo("screenheight", tt)) - h)/2)),
           sep="", collapse="")
tkwm.geometry(tt, g)
tkwm.title(tt, "こんにちは、GUI！")