To me, gratitude is what I give when I see someone put forth the effort and attention to detail needed to complete a task, and complete it well. It could be a verbal acknowledgement of the person, or a subtle nod and a high-five (as cheesy as it may seem). I know that I personally appreciate recognition for doing something well, and it makes me feel good to recognize others! The praise should not come easy; it should come a result of outstanding, hard work.
My first job was at a local gym, and I remember that even something as simple as a pat on the back from my boss meant the world to me. It meant that she saw that I was taking my work seriously and doing the best I could to be friendly and welcoming to all patrons who came to get a workout. When working for someone who is grateful for your time and is supportive of your positivity, it makes the workday seem shorter and could go a long way to retaining employees long-term.