I have used two different techniques for handling "maybe" events on my calendar. They both work reasonably well, I switch between them periodically to see if I can determine a preference. So far, either one works well enough.
Before details, I'll comment that my use of a calendar is not quite as strict as David Allen would have you believe he is about what goes on it. I do use it to block time for activities, and to flag deadlines, for example. Read Peter Drucker's "Effective Executive" on calendar usage, and you'll see how strict GTD limits the use of the calendar as an overall productivity tool. You should, for example, be able to look at your calendar for the past N weeks and identify what your priorities were during that time period. Strict GTD calendar usage prevents that technique.
Calendar "Maybe" technique 1
I enter tentative activities on my calendar with text beginning "Tentative:", e.g. "Tentative: attend concert". When I make a decision one way or the other about it, I either update or delete the entry. This works on all electronic calendars, easily and simply.
Calendar "Maybe" technique 2
Specifically in Google Calendar, I have a secondary calendar named "Tentative". I make tentative entries on that calendar, and include the "Tentative" calendar on my main calendar display in a different color. When I make a decision one way or the other, I either delete or move the entry to my primary calendar.
The second technique has the advantage of making it very easy to declutter my primary calendar by not displaying "Tentative". It has the disadvantage, perhaps, of being dependent on a particular software implementation.