@Travello, you're "allowed" to put whatever you want on the calendar. Think of
Getting Things Done not as a set of commandments but as a set of behaviors David Allen recommends as best practices. And even if you choose to do GTD by the book as I do, you'll find that there are as many ways to implement the ideas presented therein as there are individuals who choose to practice the methodology.
I realize this may seem daunting at first, particularly if you're looking for someone or something to reduce the uncertainty in your life. I've come to see this as a blessing, not a burden, though. Because the framework presented in GTD is flexible and adaptable, it doesn't break under the stresses of daily living.
@mcogilvie did a good job already of summing up how David Allen recommends you use the calendar. And I'll echo what he and others have said: if putting your work schedule and lunch breaks in the calendar helps you, then by all means do so. Those are date-and-time-specific activities.
I will add two things. First, I think of GTD as something to help manage those things in my life that are not on "cruise control." For instance, I don't need to put "brush and floss teeth" anywhere in my system because it's a habit. "Go to work" falls under that category for me as well because even though I work from home, I keep regular business hours and I make the decision about whether or not to do work after hours based on how much is on my plate at the time and whether I have any personal commitments in the evening. Eating meals is also a habit and one I don't need to schedule, because my work doesn't demand that I eat at set times.
Does your work schedule vary based on some external factors that are difficult to track in your head? Does the same hold true for your lunch breaks? Then schedule them. If not, you may not want to add them to your calendar because of the risk that they'll just be so much noise that numbs you to the things in your calendar that really are date- and/or time-specific.
Second, I have ADHD so certain things that are easy for others to manage are not for me. I got into a habit of trying to manage them on my calendar rationalizing that to some extent these things were time-sensitive -- if I don't regularly update the system we use at work to track my sales opportunities or remember to pre-book my classes at the gym ahead of time, for example, there are negative consequences. But when I couldn't get those things done on the dates I had chosen in my calendar, it was causing exactly the sort of stress and negative feelings that
@mcogilvie referred to. And it was all so unnecessary to experience that stress, because these were things I could do on a different day if needed. So I don't put them on my calendar anymore and have found other ways to remind myself about them.