You do not need gitian to build windows wallet.Its much easier and straightaway than gititan.Here are the steps to compile windows bitcoin core wallet on ubuntu with cross compile

1.Clone bitcoin repo
2.cd to depends folder
3.execute make HOST=x86_64-w64-mingw32 -j4
4.then once the depends have been compiled fully,then cd ..
5.then run bash autogen.sh
6.then execute ./configure --prefix=`pwd`/depends/x86_64-w64-mingw32
7.then type make
8.After all compile finishes.You will find qt wallet for windoes under the src/qt folder.

Thats it