Most people use a portable GPS, and I don't know how you would best use a portable GPS to assist with fuel planning.
I suppose the biggest contribution is that you know where you are and aren't going to get lost. Getting lost does have a tendency to affect one's fuel plans
But you also know your GS and your ETA, and assuming you know the fuel burn you can work out the LFOB (landing fuel on board) from that, manually. If you have 2hrs to run and your burn is 10GPH then you know you will burn another 20 gallons before you get there.
As regards extra legs, my feeling is that the best way is to plan the flight as best as one can, and execute it to the plan. Then (assuming you know the fuel burn) the fuel ought to work out.
I did some (somewhat artificial but required by the FTO) detailed fuel plans in the JAA IR training. One is supposed to maintain a fuel column in the plog. Accounting for every approach, every missed approach, every time around the hold, I managed to get the error down to 1 USG, which was about 3%. The plog form didn't have anywhere enough room so I used a spreadsheet. But paradoxically this is unworkable in "common GA" because the fuel burn is not known anywhere near that accurately. It is know to within about 20%. I was able to make it add up because I know the fuel flow to 1%, but if you know the fuel flow to 1%, and it is GPS linked to compute the LFOB, you don't need to maintain a fuel column in the plog
In fact you just look at the current FOB reading and decide if another 5 holds or whatever will deplete it too much.