Did the Subcontractor have an instruction to not finish the gullies? If you did then you can notify that this is a compensation event and any costs associated can be evaluated through the compensation event quotation process. If you didn't have an instruction and you chose to do this off your own back then however much in good faith you did this you do not have a contractual remedy.
In any situation, your only means of recovery for anything not in the tender Price is if you can get them to agree a reason for a compensation event, and follow on with quotation and assessment to get it implemented.
In any situation, your only means of recovery for anything not in the tender Price is if you can get them to agree a reason for a compensation event, and follow on with quotation and assessment to get it implemented.