You don't trim the gazis until they are dormant.
Gazi is a replacement for the Lombardy Poplar. This tree is originally from the Poplar Research Station in Turkey. it is a pure Populus Nigra, not a hybrid. This tree is resistant to canker, insects and to cold. It has grown with no problems in Minnesota. Grows to 75 ft. in about 10 years. .
After several mistakes I made, I now have about 30 gazi trees which I can sell, They are about a foot tall.