Gratitude is a feeling but a complex one that often isn't alone.
Oftentimes it is joyful but other times it can be a sadness or shame too.
You can show gratitude through actions but in the same way that not everyone who laughs is happy. Not everyone who behaves gratefully feels that way. Nor is it regardless of circumstance.
You can be grateful and see the flaws in something. Nothing is perfect after all. Life is full of grey choices. To experience one thing is to give up another. To succeed in one space is to lose in the other.
Nobody feels anything all the time.
No it is not ungrateful to defend your needs. Often people who demand gratitude are using shame to try and control the other party. I.e. I did this for you, so you must do this for me.
No, gratitude doesn't work like some kind of scale.
Gift giving can be a form of gratitude but it can be a social mechanism. The value of the things exchanged is often a function of shame and capitalist thinking. Giving back is not 100% essential for gratitude but it sure helps the giving party feel okay about it and stops them feeling like a martyr.