Here are some examples:
* Je suis reconnaissant de votre aide. (I am grateful for your help.)
* Elle est reconnaissante envers ses parents. (She is grateful to her parents.)
* Ils sont reconnaissants d'avoir survécu à l'accident. (They are grateful to have survived the accident.)
You can also use "reconnaissance" as a noun meaning "gratitude."
* Je vous exprime ma reconnaissance. (I express my gratitude to you.)
* Il a montré sa reconnaissance en leur offrant un cadeau. (He showed his gratitude by offering them a gift.)
There are other ways to express gratitude in French depending on the context, but "reconnaissant" and "reconnaissance" are the most common and versatile.