In my opinion..
Gratitude is a wonderful quality to have, that can help toward contentment in life.
A Buddha is contentment, and would not be in the act of ‘seeking’ things to be grateful for. Forcing concepts just leads to disturbance and instability in the mind. What Buddhists engage in is ‘truth’, clarity and understanding of the way things really are, including feelings. Clarity is achieved through mindfulness. If we live a moral, kind life, we cultivate purity of mind, which in turn helps our meditation bare fruit. This evokes a ‘pure’ form of gratitude/contentment from within, in tune with reality..and one that has not been ‘forced’ upon the mind (I.e. not through drugs nor imagining crystal balls nor Gods nor seeking happy thoughts to create conditions), but rather through being mindful in the presence. So, If ‘Happy’, just note.. ‘happy’.
When we begin to see the nature of reality clearly, we see within ourselves a connection to ‘all’ life, and the preciousness of it. That is gratitude (in my view). Ever visited a park and wondered why there is a preference to feeding swans, than to feeding pigeons? But when we see swans getting all the food and pigeons being left out (then how do we feel)?
If You notice on Buddha statues, there is a subtle smile (unlike the exaggerated smiles in our Facebook selfies. Probably worth looking into what that smile represents