The resource leveling command is used to resolve resource overallocations. It is not a resource optimization tool. If the resources are not fully utilized, assign them at their maximum units - then at least if the schedule is leveled they will be fully assigned when they are assigned.
You may also want to allow the leveling command to level individual assignments - that way if one resource is not overallocated s/he can work on the task independent of other works.
Regarding gaps - there really isn't any way around that. If Resource A is waiting on an output from a task that Resource B is working on - there may be times when A is just waiting without other tasks.