Gardner is basically a poor man's Michael Bourn but you're right, I'd never spend $12M per for 4 years on a center fielder who is at the age where similar players usually erode quickly. All you have to do is look at the players who compare similarly to him at age 29:
1. Max Flack - played forever and a day ago, I'm not even going to compare him.
2. Brett Butler - the only player on this list who is the exception, had his best years from 29-33 and had a long career.
3. Dave Collins - center field days were done by age 29. He was an offensive liability as a corner outfielder.
4. Roger Cedeno - career done at 30.
5. Brian Hunter - career done at 32, effectively he was done 2 years earlier.
6. Albie Pierson - career done at 30.
7-9. All played in the early 1900's.
10. Rudy Law - career done at 29.