That's a european spelling, probably passed derived the greek phonological version. In Turkish, it's properly written as gâvur. Wikipedia seems to have a short entry about it (http://en.wikipedia.org/wiki/Giaour).
Further etymology here.

Its pronounciation: gya-ur, gya as in Magyar and u as in, ermm, Ulm, yes, or as in the german Urteil. Stress on the second syllable.