Gross long-term care premiums are the total amount that you paid for a qualifed long-term care insurance policy. TT asks for gross because how much you can deduct is limited (by age) and usually a lot less than the premiums.
A long-term care policy is not a general health insurance policy. It pays for care if you cannot perform a certain number (two I think) of the "activities of daily living (e.g. feeding yourself, bathing, etc.).
This article describes what qualifying policies are and the limits for 2016.
http://www.elderlawanswers.com/irs-issues-long-term-care-premium-deductibility-limits-for-2016-15358