I have searched the internet for the answer to this, but I cannot find it: What is the difference between ‘gratitude’ and ‘gratefulness’ if any.
Both Merriam-Webster and the OED treat the words as synonyms:
gratitude: the state of being grateful
gratefulness: : the quality or state of being grateful
gratitude: The quality or condition of being grateful
gratefulness: The quality of being grateful
The abstract noun gratitude comes from Medieval Latin gratitudo, “thankfulness.”
The adjective grateful comes from an obsolete adjective grate, “agreeable, thankful,” which came from a Latin adjective, gratus, “pleasing.” Gratefulness is an abstract noun formed by adding the suffix -ness to grateful.
Personally, I prefer gratitude to gratefulness because I don’t like using -ness abstract nouns when a form without -ness is available and suits the context.
Here are some examples of what seem to me to be unnecessary -ness formations, together with the words that can be used instead:
anxiousness / anxiety
amiableness / amiability
animatedness / animation
determinedness / determination
coolness / cool (when the sense is “not warm” : in the cool of the evening)
fanaticalness / fanaticism
falseness / falsity
moistness / moisture
ostentatiousness / ostentation
poorness / poverty (when the sense is “lack of wealth”)
stupidness / stupidity
tiredness / fatigue
Bottom line: Gratitude and gratefulness mean the same thing. Which to use is a matter of stylistic choice.