This was the strongest language against gifts I was able to find, though most codes of ethics don’t ban them at all:
The AAMFT Code of Ethics mentions the term ‘gift’ in Principle III of Professional Competence and Integrity, section 3.10, in which it states, “Marriage and family therapists do not give to or receive from clients (a) gifts of substantial value or (b) gifts that impair the integrity or efficacy of the therapeutic relationship” (2001, Section III, Par. 10).