Would gold have the negative charge in gold phosphides?
Because their electronegativity difference is less than 0.4, I think the bond would have a lot of covalent character. I personally don't think it is a good idea to treat this substance as an ionic compound and assign a positively charged ion and a negatively charged.
According to a chemical supply company, $\ce{Au}$ is +3 and $\ce{P}$ is -3.  We typically write the cation first, so from the formula $\ce{AuP}$, you should get another clue that the gold ion is positively charged (sort of if you have to assign a positively charged ion).
I don't think you can assign charge signs solely on electronegativities. Atomic radius and position on the periodic table also play roles in determining which atom is more positively charged.
For electronegativity, more reference here.