Welcome back, Rachelle!
You may absolutely visit the other Disney Resorts during your stay. In fact, my family and I do this every single time we vacation at Walt Disney World. Whether we have a dining reservation at one of the resorts or we just want to check out the resort itself to perhaps stay there the next time, it's a lot of fun to do so!
While Disney transportation will take you anywhere you'd like to go at the Walt Disney World Resort, they do not offer direct resort to resort transportation, so you'll have to go to a park and then to Grand Floridian. What I'd recommend is during your day at Magic Kingdom, take a couple hours break, perhaps with a lunch reservation for a mid-day break, and hop on the
monorail. It's a lot of fun to ride the monorail. In fact, it's such a quintessential Disney experience for my kids that we always make at least one loop on the monorail each time we visit even if we're don't really have to because the kids love it so much. You can then hop off at Disney's Contemporary Resort, Disney's Polynesian Villas, and
Disney's Grand Floridian, which is, as you said, certainly a gorgeous resort.
I'm not sure when you're visiting, but if it is between now and Christmas, these three resorts are extra magical with their beautiful Christmas decorations. All three feature huge gingerbread houses, made with different gingerbread recipes. Last year, my family and I created a gingerbread tasting tour just to try the three recipes and come up with a winner. We couldn't decide because all were so tasty, but it was certainly a fun experience touring the three resorts and enjoying the monorail ride between them before returning to Magic Kingdom for the rest of our day.
Have so much fun exploring, Rachelle!
Marcy