Mehdi
You can certainly build an IPSec tunnel from an ASA5520, and you could use that IPSec to encrypt/protect a GRE tunnel passing through the ASA5520. But the ASA does not support building a GRE tunnel.
If you want GRE tunnel and IPSec tunnel terminated on the same device then you need IOS router and not ASA5520.
HTH
Rick
HTH
Rick