I think that it depends on your therapist and what the gift is. If I bought some very expensive item and tried to give it to her I think she would refuse the gift. Some therapist may not except gifts at all, I don't really think that it's inappropriate as long as it is not a frequent thing, in my two years with my T I have given her two gifts both being under $10 in price.
The reason I gave my T this gift is the magnets allow you to express your emotions in a fun way, sometimes I have troubles deciding what emotion that I am feeling in session so now I will have magnets to choose from to explain my feelings. It is an excellent question as to whether it is appropriate or not, before I gave T a gift I wondered also but it hasn't changed our relationship in anyway so I guess it's okay though others may feel differently.