Gourmand and gourmet have similar meanings, and they can be used as synonyms. However, one is typically considered to be more complimentary than the other.
A gourmand is a person who enjoys eating and often eats to excess. The original (15th century) meaning was ‘glutton’. Although it later acquired the meaning of ‘a judge of good food’, gourmand is not usually a flattering description.
A gourmet is a person with a refined taste in food. It has always had this meaning (originating in the early 19th century). You can use gourmet attributively (e.g. ‘a gourmet meal’) but you cannot do so with gourmand.
If you wish to compliment someone on their excellent palate, gourmet is the word to use.