Here is a summary from a year ago of the expectations of the scientists most responsible for proposing Gusev as a landing site.

They outlined 4 hypotheses that they wanted to test:
1. Gusev hosted lakes
2. No long term lakes but aqueous depostion from rivers and/or floods
3. Deposits are volcanic material from Appolinaris
4. The deposits are glacial

They even give a nod to Nick's White Mars ideas. Note that the eastern hills were already identified as a prime target (p.6). Given all the uncertainties in getting on the surface I think that they must be happy that Spirit deviated from the center of the landing ellipse and got as close as it did.