It IS the roast that is the difference. The only real difference in the beans is that some beans taste better at a higher roast than others, so they are more appropriate for espresso. Your Italian grocery coffee company may be using the espresso label for marketing purposes, but in general, espresso coffee beans can be the same beans that are used for "regular" coffee, but roasted to a French or Italian roast level, which is darker than City or Full City.
Since the advent of Starbucks, many roasts are much darker than they used to be. Dunkin' Donuts coffee, which is a Full City roast, used to be the norm, but now a French seems to be what you can buy.
I roast my own coffee and take it to just into the second crack which is, generally, a Full City roast...a point where the character of the coffee predominates rather than the flavor of the roast. There is more information about roasts at Sweet Marias where I buy my green beans, and reading through the site will give you way more of a coffee education than you probably ever wanted.
So, yes, you can use the coffee you have to make brewed coffee. It will probably be roastier than you would normally have, unless it is just a marketing ploy, in which case it will taste normal. Consider how long you have had this coffee; if it has been shelved for a while "normal" probably won't be all that great, since freshly roasted coffee is, generally, way better than old coffee. But as long as the oils aren't rancid, it is more likely just going to be bland.
This is called a pressurized portafilter. As you found on coffeegeek, it is designed to enhance production of crema. The Coffee Geeks look down on this device, since it produces what is essentially "fake" crema - even with old, stale pre-ground coffee.
If all you want to drink are lattes, it does not make much difference. If you want to taste decent espresso, then you must use a non-pressurized portafilter - even if you are a non-professional.
Making espresso is not a slam dunk - especially with a consumer machine like the Gaggia. You will learn to improve your espresso shots by analyzing your technique. This often is based on the appearance of the crema during and after the shot is drawn.
Since the pressurized portafilter gives you misleading information on the actual crema, you will have difficulty learning how to make a good espresso.
Best Answer
"Filter" coffee in general, is coffee beans that have been roasted and ground and is for use in coffee making machines. This makes 'fresh' coffee that may have a variety of qualities not found in instant - such as sweeter or more distinct flavours, etc.
"Instant" coffee is normally a coffee that has already been made (they had the fun already!) and is dried (usually through freezing) into a dehydrated state, hence we just add water and get coffee.
I would think that most people who have drunk proper (freshly made) coffee would prefer the flavour, quality and effects from caffeine, over that of instant. For me, instant is a back up, in case the real stuff runs out, but I feel always tastes a little woody and not as satisfying in terms of a caffeine hit. There are several coffees available now - Kenco Millicano is one - that offer both instant and some kind of coffee bean, but all I see is coffee grinds in the bottom of the cup, and not much difference in flavour.