I think Gunn would think of Xan as a spoiled pretty white boy, a softy who had it easy his whole life, growing up in SoCal.
Xander would see right through Gunn's tough exterior and see the self esteem issues and savior complex right away. (seeing is his gift remember)
They would not initially get on. However as soon as things get tough, they'd earn each others respect, and while Gunn wouldn't let it slip right away, he'd probably accept Xander as an ally... just not with the warm and fuzzies. Xander would probably accept the gruff Gunn a lot faster, aware of just how tough Gunn actually is, upon learning what he lost, and where he comes from.