GRE creates virtual tunnel in which you specify the source and the destination for the GRE tunnel, with this in mind you can use LAG/LACP to provide connectivity to the tunnel, meaning that the tunnel endpoints can be on the physical interface that connects towards the other end point.
if you are trying to get LACP within the GRE, well this is not supported.
let me know if I answered your question, or if you have any doubts or comments.
then again I don't think the MC-LAG config will be so useful with the GRE tunnel, the GRE tunnel is very limited, i don't think you can implement MC-LAG and a GRE tunnel to use it from two different MXs.
Maybe MC-LAG will resolve your problem altogether without the need for the GRE tunnel. look at the following documents: