It depends on the occasion, your relationship with the priest, and the kind of gift you have in mind. Assuming that you do not have a personal friendship with the priest, the gift should not be expensive or personal. Any gift given, whether or not you are personal friends, should not be inappropriate to his status as a priest or to his Catholic faith. As for appropriate occasions, that depends on the relationship. If this is a priest you know only on a pastoral basis, then ordinarily you should not give him gifts for personal occasions such as a birthday. Major Christian holidays (e.g., Easter, Christmas) or the anniversary of his ordination would be more appropriate.
Keep in mind also that if the priest is a consecrated religious then he may not be able to keep the gift given. All property belongs to the order and is given over to the order by its members, although some superiors do allow members to keep some items for personal use. The most appropriate gifts for consecrated religious are money donated to their order in their name or inexpensive sacramentals (e.g., holy cards, saint medals) that a superior is likely to allow him to keep for personal use.
