yes and no, Most people are talking about a gel cell when they say sealed lead acid. The whole purpose if a gel cell is to be able to use it in any direction. ie sideways without any ill effects. Your maintenance free batteries like the ones for your car are not gell cells. They are wet batteries that use some techniques to get the gasses reobsorbed without having to vent the gasses.
Batteries are basic in principle but can get kinda complicated when you start to get technical with them. A regular sealed battery will eventually vent some and will go bad because the level of the electrolytic acid gets low.
Charging is also another consideration. With a car battery, it is designed to put out a bunch of power at one time and then pretty much just sit there while the alternator does the work. A UPS battery is designed to be discharged to a greater exent and then recharged. It is not designed to put out a whole bunch of power at one time though. This starts to get into plate thickness and design and function. You've probably noticed that a marine battery is a little heavier than the same size automotive battery. That is because it has thicker plates and thus more lead in the plates. The thinner size of the auto battery allows the energy to flow out of it a little faster.
To just answer your question though, no they are not the same. A sealed battery cannot be turned on its side while a gel cell can. The sealed battery does have some of the same disadvantaegs though. A sealed battery is more likely to pop if it is over charged. This kinda goes back to the 18 volt thing. Too much gassing inside of the battery and it cannot reobsorbe the gasses like it is designed to.
One of the reasons you need to keep the water level of your car battery where it is supposed to be is to reduce the amount of the gasses inside the battery. NEVER smoke around a charging battery. The gasses that come out of it can explode if ignited. When I was younger, I worked at SEARS in the automotive department. We had a room that was for all of the batteries. Charging and just storage, waiting to be installed or sold. The charging batteries were the one s that you had to be careful around. You could smell the gasses coming out of them (as many as 20 to 40 batteries charging at the same time). While I worked there, a fellow in Florida lit a cigarette in the battery room in the shop that he worked at and had to go to the hospital. They said it blew him right out of the battery room kinda like in the movies. (our insurance company made them stop letting us smoke in the shop after that. With all of the gas and cleaners and stuff, somebody got hurt bad smoking around a battery) aint that a kick in the head.