Why did he hire a lawyer when he has Gonzales?
Because the investigation involves the adminstration; Gonzales can't represent him personally.
I dunno. Perhaps since so much of the executive branch conceivably could be targeted in the leak probe, even Gonzalez's office(however unlikely), it was wise to get someone completely independent. Or perhaps it simply has to do with the intricacies of when and where the WH counsel can represents the president vs. Bush the citizen, and avoiding any limits or potential avenues of future accusations and legal diversionary techniques.