How are the gross clicks calculated?

You can review the information about gross clicks and how they are calculated in the Clickthrough Performance Report.

Gross Clicks are the total number of clicks the individual link receives.
Total Gross Clicks are the total number of gross clicks within the entire email.

If there are 10 links in an email, and 1 out of 10 of those links was clicked twice while 9 of the links were not, then the Clickthrough Performance Report would show 2/100%. This is because that one link was clicked twice and received 100% of the activity within the mailing. The Clickthrough performance Report can best be thought of as a popularity contest.