If all you want out of a tire is high mileage that's easy to do with a cheap tire. If you want no flats in addition to the high mileage, that usually will require a better tire.

No matter what you get, you're giving something up somewhere. Usually high mileage comes at the expense of grip. For a lot of people that's a good trade-off because they usually don't ride near the grip limits of their tires anyway. High mileage with good flat protection usually means a stiff and heavy tire. That's OK too for a lot of people.

Whoever recommended the Gatorskins to you was recommending a tire that was relatively light, would roll well on good pavement and had decent puncture protection for a moderate price. The Marathon Plus won't meet those criteria, but if those aren't your criteria then that might be OK.

As for puncture protection, there are two basic ways to stop a puncture: (1) a thick, hard bulk of material, or (2) a belt of very strong fabric-based protection (such as Kevlar, Vectran, etc.). The thick, hard barrier pairs well with high mileage tires because the people using those tires generally aren't overly concerned with performance. The Marathon Plus is top of the heap in this category. The fabric-based protection gets paired with lightweight tires for people who are looking for performance. A lot of the fabric-based belts do very well, but they tend to be vulnerable to very sharp shards which can get a point between the weave of the material and spread it out. Depending on how the tire is constructed, it may also be vulnerable to side-wall failure. The fabric-based system is also more vulnerable to pinch flats because it is designed to flex with the tire.