When I started making wine, I made what is known as Balloon Wine. Balloon Wine refers to the use of a large Balloon for keeping a artificial atmosphere in the wine making vessel, usually a gallon jug or bottle, instead of using a regular fermentation lock. It is a cheap, but effective way of starting out.
There isn't anything wrong with using a balloon if you are only making a gallon, but balloons are single use. For a few bucks, you can buy a fermentation lock and use it over and over and over again. There is also a chance that the balloon can break. When that happens, the batch is basically shot - unless you have another balloon ready at the instant something goes wrong. Any air leakage into your bottle or jug can lead to oxidation, which ruins any batch of wine.
If you are trying your hand at wine making, just to see if you might like the hobby, using a balloon may be the way to go. Wine Hobby shops are usually listed in the Yellow pages or on the Net and will have plenty of fermentation locks and other supplies available, should you want to jump in with both feet.
I see no advantage in using a balloon instead of a fermentation lock for gallon jugs, when you consider that you have to search for a balloon big and sturdy enough to last up to 5 weeks with alot of Carbon Dioxide inside.
Wine making is fun and can be challenging. Practice makes perfect, and using a balloon can be a starting point for some. But for various reasons, I would recommend using fermentation locks.