I think that person is NOT from Gokuldham society because of two reasons,
(a) Kothay had already lied before, why should we trust him now?
(b) Kothay found that the biggest strength of Bhide was Gokuldham society, so he intentionally told this lie that someone from GK society informed him. This trick will help him as GK members will start mistrusting each other and therefore their strength will decrease.
kothay must have hired someone (outside GK) to keep a close eye on GK members. He must have hid somewhere during the GK meeting and informed kothay about it.
I don't think kotay got any hint when GK members were dancing because he was quite tensed even after the call. kothay became confident only after his hired person informed him about the plot.
Let's see what happens...)