In Katniss's memory, Gale's lips and breath have a pleasant citrus scent. This is her main memory of the single kiss they shared, at a time when, due to political reasons, she had to appear to be head over heels in love with Peeta Mellark.

Katniss comments that there is no reason to compare this kiss with the many that she has shared with Peeta, because it was a totally different experience, not based on acting, trying to stay alive in the arena, or playing a part insisted upon by President Snow.

The next time Katniss and Gale saw each other, both acted as though the kiss had never happened, although Katniss had prepared a speech about how she would never get romantically involved or married due to the situation endured by residents of the twelve districts of Panem.

I believe you are referring to Chapter Two of Catching Fire, the second in Suzanne Collins' The Hunger Games series, in which protagonist Katniss Everdeen remembers the kiss she shared with her lifelong best friend Gale Hawthorne. The memory is triggered by a threat from President Snow, who makes it clear that he is aware of Katniss and Gale's complicated relationship even though he should have no way of knowing that they are anything more than the "cousins" they are pretending to be to avoid public scrutiny. The kiss happened on a day that Katniss and Gale spent out in the woods, following a picnic Katniss prepared of chicken, cheese, bread and oranges. While Katniss does not specifically state what Gale's breath tasted or smelled of, she does strongly associate their kiss with the scent of oranges, which lingers from their picnic.

Interestingly, President Snow's breath features prominently in the following chapter, in which Katniss realizes that it smells of blood.