Just my two cents.... I'd stay away from the gel batteries. The main reason being you can not charge them very quick and if you over charge them they are garbage, generally speaking. If you want a sealed type battery then go with a more expensive agm. The agm has all the advantages of a gel and none of the disadvantages. Or stick with the flooded style batteries if you have the room and don't mind checking the water levels. (it's really not a huge deal as long as you have the room to work)
Me personally, I went with sam's club 6 volt golf cart batteries. $78 a piece. For my first set of house batteries I did not want to spend a fortune on them just in case I ruined them.
Also make sure you get a nice quality 3 or 4 stage charger. And keep the battery (s) charged up. Try not to let them fall bellow 50%.
If you want to be a little more accurate, add up all the watts of everything you run off the battery. Post up the numbers and we can help do some math equations and show you how many batteries and what size, etc. Then you will have a good idea as to how long they would power what you need.