Yes, just add him as one or those then add the other conection.
If you added him by adding a spouse to your great aunt then go to your great uncle's father and add him as the father
If you added him by adding a father to the father of your great uncle go to him or your great aunt and select Add partner/ex then Partner with person already on tree, search for the other and finally go to the Partners tab and select Edit [name]'s details there you can change the type of relationship between them
