What’s the reason for a GAP ?

The mission of the FMS is to fly the sequence of waypoints that is entered into the flight plan.  As long as the flight plan is an accurate representation of your clearance, everything should work out just fine.  The problem here is that when an expected clearance is entered, the flight plan no longer represents your actual clearance.  But that's where the gap comes to the rescue.  Remember the purpose of the gap is to tell you that the FMS doesn't know how you want to get from the waypoint before the gap to the waypoint after the gap.  If you receive clearance for the approach before you have reached the last enroute waypoint, then you can just close the gap and the FMS will happily sequence onto the approach with no further action from you.  Or, if your clearance is direct to the IAF, you would just do -D->.  But, if you are getting close to the last enroute waypoint and haven't yet been cleared for the approach, then your clearance is still to the airport.  In that case, just enter a leg to the airport immediately prior to the gap and the flight plan is again representative of your clearance.  Then, when you're cleared to start the approach, you would just -D-> the IAF or whatever you're instructed to do to start the approach.

I disagree; "expect RNAV 23" as used above is not a clearance and does not change any previously assigned altitude, speed, or routing clearance. It is purely informational for the pilot's planning purposes and lets the pilot have a general idea of what to expect and plan for when the time comes that ATC actually issues a further clearance.

As far as whether a pilot will fly at least part of the final airport leg or not, that depends on a myriad of factors; length of that leg from previous waypoint, direction of airport leg versus approach direction, local airspace and ATC procedures, etc. Local knowledge of these will factors will certainly aid the pilot in deciding whether the dropped airport leg should be inserted into the flight plan or use an alternate technique to handle that potential routing.

Staying ahead of the airplane is what this is all about.

At the beginning of a flight, my last enroute fix is just before the destination airport in the flight plan.  Before starting my descent, I've briefed an approach I expect to get and loaded it into the IFD.  Often, that creates a Gap if the approach starts at a fix not in my route.  Not a problem, it's to deal with later.  I've also reviewed some other approaches I might get, so that I won't be behind things if I don't get the approach I expect.

As I get closer, I usually am told to expect the approach I planned for, so I change nothing.  But if I get a different one to expect, I delete the first one and replace it with the new one.

All this is done to reduce workload when I get the actual clearance for the approach, when things get a little busier.  If that clearance is for the approach I've loaded, I delete the Gap if there is one, proceed to whatever fix I've been cleared to, and fly the approach.

At no time do I consider that I need an approach clearance in order to load an approach into the IFD.  I don't even need a controller to tell me what to expect.  I load it myself after reviewing the METAR and surmising which approach I'd prefer.  And if the controller says to expect some other approach I don't like, I'll ask for the approach I prefer.  That doesn't happen often, but when it does, ATC will often accommodate.

Nice. Got me there. 

Bob Quote=The bottom line for me is:

1. Entering the approach in the IFD is indicating that I expect a new clearance on how to get to the airport other than how I am currently cleared.  For me, that expectation includes a decision about flying the last enroute leg to the airport.

Very good explanation. I can understand where your coming from now. Not totally onboard but makes sense. 

Bob Quote=5. I talk too much, analyze things to death, and make points that are not always germane to the point, and thereby just obfuscate the issue!  

No worries Bob. Keeps things interesting. We’re all here to learn.