Questions about example sentences with, and the definition and usage of "Gochisousamadesita"

A: × We say gochisousamadesita after the meal being thankful to the meal.
✓ We say gochisousamadeshita after a meal to show thanks for the meal.

"After a meal" sounds more like you're talking about all means, while "after the meal" almost sounds like you're talking about one specific meal. More correct to use "a".

At minimum, you also need to use "to be thankful". It's not grammatically correct to just say "being thankful to the meal" at the end of a sentence like you did.

I've changed "to the meal" into "for the meal", as it sounds more natural, but it does change the nuance. "I'm thankful for the meal" is me telling someone else that I am thankful they gave me the meal, while "I'm thankful to the meal" is me deliberately thanking the meal for existing.

I learned ごちそうさまでした as something like "thank you, that meal was excellent", so I can't know if "to the meal" was intentional or not.

If I were speaking, I would probably say this sentence as, "To show our thanks for a meal, we say gochisousamadeshita after eating", but that means the same thing. Just flows a bit more naturally, I think