Firstly is Illegal for him NOT to provide you with the basic safety gear, this includes harness, rope, biners, helmet etc etc etc. Other pretty bits of kit are up to you, but he MUST provide the basics. Department of Labour rules.
As for the GST. A little tricky. Are you paying for it through your hours?
For example if your wanting a bit of gear and it costs $20 and you make 20 an hour before tax, are you just taking that hour off your wages sheet and hes buying you the gear?
Now hes keeping the receipt and claiming deprecitation of that bit of gear. And hes claiming the GST back. Yeah hes getting the much better deal but your not having to pay that hours tax and for that hour getting more bang for your buck.
Its not strictly fair, but its a big shady to so you cant exactly go to the IRD or DOL about it.