tunahp
May I submit you are looking at this far too simply--every airplane is the result of many, many compromises each decided by the designer's brief as to what performance, size, range characteristics the marketing people think will be attractive to buyers.
The Falcon three-engine decision was originally made in the '70s when the very idea of 2-engine overwater operations were inconceivable. The first airline ETOPS routes weren't extensively used until the mid-80s. Three or four engines were considered essential and Falcon was offering a product that was "airline grade" as the term was then. Additionally, engines tend to drive airframes, not vice versa. The available powerplants, in the thrust range needed drove some of the decisions. In brief, Falcon chose three engines because it was considered "necessary" to market an overwater, airline-grade business jet and the TFE 731 was the only available engine. The didn't plan on or anticipate the problems TFE-731 would have.
On to wings. The GLEX and its competition the G550 have very similar wing spans--the "footprint" on the ramp of the two planes is very close and they have basically the same engines. The GLEX has about 100 square feet less wing area, so has a slightly higher aspect ratio. It has a very sophisticated high-lift slat and flap system, combined with a powerful rudder, the wing allows it to have low Vmcg and Vmca; thus low speed numbers in the take-off and landing regimes--about 15 knots less than the G550 at similar gross weights. Wings are the product of very complex fluid dynamics well beyond aspect ratios, span loading, wing loading, fineness ratios. Yes, high aspect ratios deliver less drag, all else being equal which it never is. Virtually, all planes that operate at relatively high cruise lift coefficients will have high aspect ratios--B777 or a glider. Similarly, longer wings may weigh more for a given area, but cleverly place four engines across the span and that can be overcome--see B747. Boeing is famous for designing wings with efficient span loadings and, hence, good wing bending relief.
The added wing area of the 6,000 pound lighter G550 allows it to come up with similar runway performance, when compared on similar trip lengths. This performance despite a very simple flap system and a Vmcg about 20 knots greater. The lighter weight of the G550 allows it to operate over about 6% greater non-stop legs at the expense of operating about 15 KTAS slower.
The GLEX has a much roomier cabin, about 10 inches wider, flies about M.03-M.05 faster, but the overall fuel burn is a bit greater.
As you can see, the designers of the two planes had very different briefs. The G550 was a derivative of the G IV while the GLEX was a clean sheet design. So the list of compromises were different, they are marketed a bit differently and buyers choose between the two planes based on those different compromises. Some want a big cabin and pay the price in fuel and slightly less range; others want range, the reputation and are happy with other features inherent in buying Gulfstream.
What all this says is that aircraft design is a very difficult and complex process and comparing what are rather simple outward characteristics, like aspect ratios and engine counts, is going to be unrewarding.