I use SiCl4 and Ar in equal proportion.  Have had maybe 100 selectivity
for GaAs over Au.  Gold's my etch mask of choice!

Brent Garber wrote:
> I have never used gold as a dry etch mask.  You are not limited to Cl2
> to etch GaAs.  Simple F12 works too, but I don't know if it will attack
> gold.  For very long etches I will do a Ni lift-off as a mask to protect
> my resist.
>
> Brent