Is it unethical to give a gift to my professor who might potentially write me a LOR?
Your first sentence tells us that you are still a student in the institution where these professors teach.  If you are currently in classes with any of these professors, or there is any possibility that you will be in their classes in the future, do not offer a gift, either prior to a request for an LOR or as a thank-you for having written one.

Giving anything of value to someone who will assign a grade is almost certain to make the professor uncomfortable and, although very unlikely, could cause the professor a great deal of difficulty.

Finally, consider that writing letters of recommendation is part of a professor's job.  You shouldn't think of doing anything that will seem like compensation for a professor having done the professor's job.  

The things I value most from students for whom I've written LORs are notes telling me that they've been admitted and thanking me for my part in making that happen.  A particularly nice one was written on a postcard depicting the former student's new university.

Keep the goodies or share them with fellow students.  Send the professors a nice note after you've graduated and been admitted to graduate school.