Not really. OmniPlan is a pure project management app - it handles schedules, dependencies, resources and costs and is overkill for most task planning. The Gantt chart it produces is simply a representation of what might be complex underlying information about all the above factors.
SheetPlanner is (among other things) a structured task management tool - it’ll do you a timeline (Gantt chart) based on how you’ve put your tasks together but it does a multitude of other things that OmniPlan doesn’t do. It’s more in the line of a Personal Information manager (remember those?).
I’m experimenting with SP as a way of doing the high level planning for projects I run - then I use OmniFocus for my own task management and Omniplan (or Merlin) for the formal project planning and management. SP is a much more friendly environment for the early stage thinking than any of the others I’ve mentioned.
In my own personal ecosystem, it’s closer to Curio than anything else.