Every single VPN provider offers L2TP over IPSec. What is the reason GRE over IPSec is never offered ? Is there any downfall ? It seems to be exactly the same.
So I have found the answer a year later while I was reading about IKEv1 vs IKEv2.
L2TP/IPSec makes it possible to use a username/password, because L2TP is built on top of ppp.
With GRE, that wouldn't be possible to authenticate with a username/password.
In IKEv2, you can use a username/password directly, so there is no need for L2TP.