You get free gas at the beginning of the game and you can also find it on missions, with higher level youll find 10 gas per mission pretty often, there are also books that increase gas drop chance. so in my opinion this model is pretty fair and doesnt urge you to buy gas for real money.
I wonder if only my research was useless at the end? And if so how could i achieve an other ending of the game? I mean my Zombies play guitar, brush their teeth, eat apples, play with cats, listen to radio, read books .. what else could you ask for ? Of course i know how to use weapons on them also..
 "The Last Stand: Dead Zone"){kind=icon}
 "Infectonator 2"){kind=icon}
 "Decision 2"){kind=icon}
 "Lab of the Dead"){kind=icon}