Gross ticket income refers to the amount paid by audience members to access your group’s event. Therefore, regardless of the breakdown of the ticket price – for instance some of the fee goes back to the group and the rest as an admin fee to the ticket provider - it would be the total amount of all tickets purchased.
As an example, if the audience pays £15 per ticket and you sell 100 tickets then the gross ticket income would be £1500 irrespective of how much of that fee goes back to the group and how much goes to a ticket provider.