What is a multi-project Gantt chart?
The Ganttify plan for the integration I'm using shows the option for multi-project Gantt charts. I have more than one project in Trello/Basecamp. Do I have to choose this plan?
No, you're not required to choose a plan that supports multi-project Gantt charts. A multi-project Gantt chart in Ganttify refers to the ability to add more than one project in a single Gantt chart to get that big-picture overview of multiple projects at once. If you're working on a single project or simply don't require this feature, you won't have to choose a plan that supports this.
You can have dozens of projects in the integration you're using and still use single-project Gantt charts in Ganttify. All the projects you have will always show up in your project list.