Can a contract iphone 6 be used as a gophone?

Yes, any AT&T compatible phone can be used as a gophone.

You said they were contract phones....was the contract paid off?

you will only be able to activate these phones if they are not on another customers account.

Go to an ATT store to get SIM cards and activate the phones on GoPhone. 

The phone can still be activated. That way if a person buys a phone and then sells the phone and doesn't pay for the phone, it can still be used but can only be used on the AT&T network as it will not be unlocked until it is paid for.