The liturgical laws of the Church do say that a priest is always to use the prayers and gestures proper to his own liturgical rite–this is articulated in a number of different places.
Something like this would depend very much on the context. A Western priest would certainly not be within the norms to use this gesture in the Mass or any other formal liturgy of the Church. On the other hand, if it is a casual situation–such as a person simply asking a priest to bless a medal or cross in a private setting, it would not realy be a problem.
You’ll certainly find a liturgical norm which says “a priest is to use the gestures, vestments, etc. of his own church…” or something very similar. I know that these are out there (but I’ll hold off on looking for references for the moment—see the rest of the post). But a norm/law which says the specific words “A priest of the Latin rite may not use the hand-gestures proper to the Eastern Churches when imparting a blessing” seems like it would be a little too specific (in other words, the part is already contained within the whole).
If you give your question a little more context, posters here might be able to provide some better answers. Are you looking for an example which specifically prohibits the gesture, or are you looking for Church laws which say that the various rites cannot be mixed? Are you looking for a reason why the pope would be an exception?
As for the pope, because he is the pastor of the universal Church, he may function as a priest (sacerdos) of any of the rites since he is a member of all of them. But even the pope does not “mix” the different traditions (I recall that John Paul II once did so, in a visit to the Middle East, mixing the various traditions in one service, but that was an exceptional situation).