When we say GaAs LED we mean in stead of Silicon or any other basic semiconductor material GaAs is chosen for substrate where you diffuse or do anything else to make it p-type or n-type. There are many reasons why in stead of Si, in spite of availability and all virtues we associate with Si, cannot be used in LED. Mainly the wavelength and also indirectness of band-gap are the two major issues. But basic material may be GaAs or any other suitable compound material depending on the wavelength requirement