Had this myself recently and my understanding is that if, as in my case, the number 4 demand pump was U/S the take-off performance must be calculated on the possibility of a failure of the number 4 engine. If this happened and with no number 4 system available the wing gear would stay down. The effect of this in the second segment is obvious with the climb performance the most limiting. ( I had a long runway). I assume the change in VMCA is an aerodynamic one because of the dangling bits in the second segment. I ended up off-loading 10 tons. I havn't got the manuals handy at the moment but I think there is a adjustment to the VMCA with the brakes de-activated when not using the de-activation tool. With capped lines the gear has to be left down for a few minutes to wait for them to stop spinning prior to tucking them away again having a large effect on performance.
Looking at your post again you mention VMCG? Are you sure it's 'G' and not 'A' If it's the ground one a reduction in roll or rudder performance might be the answer but to be honest with all the back-up I cannot see why.