Throughout the movie, Gretchen harbored resentment towards her mom for her past actions, which impacted their relationship. As she gains understanding and finds closure through the journey, Gretchen waving goodbye to her mom as she leaves the town symbolizes her letting go of the bitterness that had been holding her back.
This gesture also portrays Gretchen's realization of her mother's perspective and the challenges she had faced. It suggests a sense of empathy and compassion that Gretchen developed along her journey, allowing her to forgive and move forward.
Ultimately, Gretchen waving to her mom in the final scene represents the healing of their relationship. This moment brings closure and a newfound sense of connection, signaling the beginning of a mended and stronger bond between mother and daughter.