Can anyone help me to express this condition in Gams?

Question

I have this condition and want to express it in Gams! But Really dont Know How to do it!

  If (t'+L(d)*(1-alfa)=t) ---> Y=1
  Otherwise               ---> Y=0

Y is a Binary variable, alfa is a continuous variable L(d) is a parameter, t and t' are time sets.

Thanks for your help!