put the code to draw and the code to check if the button is pressed in the same loop. i.e.
repeat
gcls
for i=1 to 20
rnd1=rnd(400)
rnd2=rnd(240)
gfill rnd1,rnd2,rnd1+10,rnd2+10,rgb(20,rnd(100)+155,40)
next
gputchr 20,20,"Le Game!",rgb(20,20,235),
vsync
until button()==#a
2そうだね プレイ済み