Cash in the Z1-2 ticket (~£650), buy an annual Z1-3 (£1208), cash it
in August (~£900), buy a new Z1-2 (£1028), cash it in in January (~
£600). Get the timing right and the net outlay by you is well under
£100 and you don't lose your Gold Card benefits. Obviously it requires
you to have some spare cash available.

Alternatively, the suggestion of a Bowes Park-Finsbury Park rail
season ticket is a good one (£46.90/month or £114.60 pro rata -
remember you can buy arbitrary length season tickets and Travelcards
at the monthly rate).

U