Which Alex Gonzalez do you have? The Cubs' SS got off to a good start this year, and he has surprising power when he's on top of his game. He has the potential to be a top 10 SS (although not much better than that). The Marlins' SS, on the other hand, doesn't look like anything special to me. I'm kinda hoping Andy Fox gets the chance to start later in the year.
Julio Lugo or Royce Clayton would probably be the best option if you're not confident in Gonzalez's ability to produce. But if you have the Cubs' SS, and you trust that he'll hit 15-20 HR's with about a .250 or better average, then neither one is much of an upgrade. And both have little upside.
If you're looking for a possible sleeper, I'd go with Cesar Izturis. There has been a little talk about him, and he does have potential. He could potentially hit about .280 with 30 or more SB's. Craig Counsell is also a respectable option, considering his eligibility for multiple positions, but again has little upside. On the other hand, he'll likely post the best BA of any of your options.
Basically, if you have the Cubs' Alex Gonzalez, I'd pick up Izturis. If you have the Marlins' Alex Gonzalez, I'd drop him and pick up Counsell and any one of the other 3 mentioned above.