Currently, the system forces donors to choose a ticket in order to use the additional gift option. For donors who only wish to make a gift and not attend, they cannot enter a dollar amount in the 'additional gift' field without choosing a ticket option. The only workaround is to add a ticket option of 'Donation', have the donor "buy" 1 of those for $0, and then enter the gift amount below. 

It would make more sense if there were an option to select/de-select if ticket options were required, so the 'additional gift' field could be used without "buying" a ticket.