Some of his tactics may be defensive, but the team is winning under Dhoni and he is actively contributing to those wins as shown by his average of 90+ in the games that we have won and that's what matters the most to me as an Indian fan. I have absolutely no qualms about Dhoni captaining the side at present and in the near future. Don't fix something which isn't broken.
To add to what I've said, you can't jump into conclusions about a particular captain on the basis of only three matches against a side which looks weak and debiliated by injuries and out-of-form players. Let Gambhir prove his mettle against the better and stronger sides before we moot his name as a successor to Dhoni. Moreover, Dhoni and Gambhir are both 29. So, technically, there's no chance of Gambhir ever becoming a permanent skipper if Dhoni continues his winning streak and sound form. Virat Kohli is a more plausible name if we are to discuss about future captains.
In spite of what you've said, Gambhir has fair amount of chances to Captain the side in many occasions. Partly predicting the future, Not always the same captain would be up there to work on the increasing number of games the team is getting to play. Nothing's par dhoni. But good thing Gambhir's also proving the responsibility of being the captain, as for his form and for the team.
His tactics were not defensive, He kept constant of one bunch of his players and experimented with the other bunch, which is a good thing. He kept his 4 mainstream bowlers doing the damage, and the part timers getting an equal share as the main bowlers in the lateral stage of the innings, which is far enough for NZ players to tumble for.
But the bottom line, He's been tested out, worked good. Now the selectors can fancy the win-win choice they have. Dhoni will feel comfortable having a good leader beside him so good change for India.