Stainless Steel Grade: There are many flavors of stainless steel. 304 stainless is the most common (about 50% of the worlds production). It is used in everything from cars and cookware to buildings. What you need to know is that there is magnetic (400) and non-magnetic (300) stainless steel. 400 series stainless steel is not as durable or corrosion (rust) resistant as 300 series. If you want durability in your stainless steel gas grill put a magnet on it. If the magnet sticks, don't buy it. Some manufacturers use a mixture of different metals so put that magnet all over the grill to test it all.
Thickness of the metal: I wouldn't suggest you take a pair of calipers with you to buy a gas grill, but there are a few ways to get a good idea of the thickness of the metal in a grill. Thicker metal will not discolor as fast as thinner stainless steels. Thicker is better. You can tell the thickness of the metal of a stainless steel gas grill by the overall weight of the grill and how hard the metal feels. Don't be shy and press the grill in the middle of the hood. If it doesn't budge the metal is probably pretty thick.
The finish of the steel: Most rust gets its start in imperfections in the surface of metal. Tiny little holes let moisture and chemicals sit and eat away at metal. Stainless steel, though resistant, isn't immune to these problems. When looking at a stainless steel gas grill, feel the surface. It should have a clean finish to it. There are several finishes to stainless steel, but poorer quality stainless should feel uneven. This metal is raw from the factory and not finished. A finished surface may be brushed or smooth, but it should feel even.
Assembly of the parts: Stainless steel needs to be properly assembled to make the most of the properties of the metal. Spot welding will give corrosion points to the metal and when you push two pieces of stainless steel together they will fuse over time. The best stainless steel grills are completely welded. This is an expensive process and isn't available on lower end grills. What you want to look for is that all the pieces come together right and they are fastened together will stainless steel fasteners or welded. Generally pop-rivets are a bad idea and if it isn't welded look for bolted joints, but check the bolts with your magnet. Steel bolts will rust quickly.
If you end up with a stainless steel gas grill you need to take care of it. Keep it clean, keep it covered, and keep chemicals away from the metal. Lawn fertilizers are very corrosive agents and can ruin even the highest quality gas grills.