If 6 are too heavy, then use 4x 6v GC2, or 4x real 12v deep cycles (not the marine type). Get them large enough for the amp-hours (AH) you feel you need for your off-grid usage. More AH means more weight, though, so if weight is your limiting factor you are going to have to live with fewer AH. AH capacity is directly related to the amount of lead in a battery, so a battery with more AH always weighs more.
AGM or not is a different question and basically a no-maintenance vs maintenance discussion. You pay a lot more to get the zero maintenance that comes with AGM.
Trojan makes some dandy 12v deep cycles in both flooded cell and AGM designs and either will give you many years of good service. My Trojan AGM 31's (Group 31 size, 102 AH each) are 8 years old now and still doing quite well.
And 12v batteries gives you the option to use 5 instead of 6, if that helps the weight issue.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL