Where can I find the Gordon Lair?

A man of his status could only reside in a fortress or castle or something, surely.

Or the remains of one.

Have you been visiting the Ruins regularly?

You must have noticed a door in the 1600s.

If the door is locked, you'll need to do something to unlock it.

In fact, you have already completed two out of three steps.

Not surprisingly for someone like Mr. Gordon, the lock to his lair is on a different door altogether.

Or, rather, the locks.

Three of them.

Unlock the third lock in the Fruit and Veg cellar to open the door to Mr. Gordon's Lair.