With Garza's stock on the rise, I think the Cubs could certainly get a better haul for him. Gyorko would be an interesting prospect, although I'm not too high on any of the other Padres' farm hands. With Cubs still need to consider resigning Garza long term, especially if they think what he'll fetch on the open market will fit with what they're willing to spend this offseason. The front office has made it clear they're willing to spend money during the offseason, so it wouldn't be out of the question. If Garza settles in on a deal similar to the one given to Anibal Sanchez (4 or 5 years with a team option at $14-16MM anually), I could see that working in the Cubs' favor.
I think we're still too far away from the deadline to get too deep into rumors. There will be more injuries on contenders that will help drive up Garza's value. He just needs to do his part and keep pitching well. If that's the case, it may be almost impossible to keep him, in regards to both what he could fetch on the open market and what a team would be willing to do in order to land him.