PA32 pilot reports possibly entering FUL Class D airspace due to its depiction on his GPS as a circle; when in reality it is a rectangle.

Narrative

On a personal flight to LGB; I picked up VFR advisories; as I like to do when flying in the LA basin. Southwest of FUL; the Approach Controller handed me off with the instruction to stay clear of FUL airspace. I was above at about 2600 FT. After a few moments; my GPS showed me outside of the southwest 'quadrant' of FUL airspace; so I started to descend. I then remembered that on my GPS; the commercial charts depict FUL as a circle when it really is rectangular in shape (confirmed at the time with a cross reference with my sectional). By the time I checked; I had already called up LGB Tower and was within their airspace. There is a chance I might have inadvertently entered a corner of FUL airspace during the transition.1. No matter how familiar you are with a route; if you haven't flown it for a while a pre-flight briefing is always a good idea.2. In the past I had never been handed off this early; or given this instruction. It's good to anticipate; but you have to be prepared for changes. I might not always be cleared through everyone's airspace 'as usual.'3. I think it would be very beneficial for the commercial chart maker to correct their electronic charts to depict the airspace as it is on the sectionals.