A GFE is a booking where the escort brings more intimacy, mainly through body language and touching, into the booking.
In regards to what it entails, while there are some common factors, each escort may have slightly different boundaries and limits on what her particular GFE includes.
In a standard booking it is common to have sex, to do oral on him and a massage if wished. Some escorts will have GFE already included as their standard service. Their standard price will normally be higher than an escort who does GFE as an add on service rather than part of it.
This is a list of things that may be a part of the GFE service, but it should not be taken as absolute. What an escort does or does not include also comes down to what she feels comfortable providing.