public Builder setGravity(
          org.box2d.proto.Box2D.PbVec2.Builder builderForValue) {
        if (gravityBuilder_ == null) {
          gravity_ = builderForValue.build();
          onChanged();
        } else {
          gravityBuilder_.setMessage(builderForValue.build());
        }
        bitField0_ |= 0x00000002;
        return this;
      }