Yes, definitely it is part of and important of happiness. If you have everything and you are not grateful for having it,it is useless.For less people, when they are grateful for what they have even if it's not a lot, but when gratefulness exist, happiness arises. The act of gratitude is part of our life for having what we have while others can't have what you have. It is a simple thing to achieve the happiness. For example, you see those children on the street, they look so dirty while playing with their friends,but you can see the smile in their faces. That is the innocent gratitude that they are happy of what they are. While we see them as poor, but we thank and being grateful that we have what they can have and this will encourage you to look at and help those children to give smile on their face.