Jonas was out at night, stole food, and stole his father’s bike.
Jonas has broken three rules when he escapes with Gabriel. In Jonas’s community, when a person breaks three rules, he is released. Jonas counts the rules that he breaks as he escapes, knowing that he is writing his own death sentence. Gabriel is already condemned.
It was not safe to spend time looking back. He thought of the rules he had broken so far: enough that if he were caught, now, he would be condemned. (Ch. 21)
The first rule that he breaks is being outside of his house at night, which he describes as a “major transgression.” Second, he takes food with him, just leftovers, but he describes that as “a serious crime.” Food is tightly controlled in the community.
The last thing he did was take his father’s bicycle. The reason he took his father’s bicycle instead of using his own was that he needed to have a way to carry Gabe.
He had hesitated for a moment, standing beside the bikeport in the darkness, not wanting anything of his father's and uncertain, as well, whether he could comfortably ride the larger bike when he was so accustomed to his own. (Ch. 21)
Jonas had originally had such a good plan. He worked it out with The Giver, and it involved his help. However, when he found out that Gabe was going to be released, Jonas had to escape early, at night, with next to no plan at all. The Giver would just have to improvise when Jonas was found missing.
The fact that the community would release someone as important as Jonas is proof that they take their rules very seriously. Three rules broken means release. Jonas seems to feel that this applies to him too, even as important as he is.