To create a Gallery with multiple rounds of voting, you'll start by creating a standard Gallery. You can reference this article, or video walkthrough to do so.
Once you have created a standard Gallery, let it run as normal. Let users enter their submissions and have enabled voting during the proper timeframe.
Once you are ready for the next round of voting to start, it's time to remove the submissions that won't be in round 2. Start by navigating to the "Submissions" tab within the setup of the Gallery. Next, go to the "Approved" section. If you are only having the submissions with the most votes move on, you can easily see the highest voted submissions by using the sort options on the right to sort by "Highest Votes". Then you'll reject any submission that won't be moving on to the next round by clicking on the "Reject" text on each submission.
Once you have removed all of the submissions that won't be in the next round, you can reset the votes of each submission by hovering over the amount of votes on each submission has and clicking the "Reset" button. If needed, make sure to extend the voting schedule to allow users to continue to make votes.
If you have more than just two rounds of voting, you'll just need to repeat the process.