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 just 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.