By default the IntelliGantt Web Part will allow you to upload files as attachments to task items.
You can also link task items to a Shared Documents list by creating a Lookup Column called 'Documents'.
When you drag and drop files from your local computer and drop them on a task item the files will be uploaded to the Shared Documents listed pointed to by the new 'Documents' lookup column.
The dropped file has been added to the 'Shared Documents' list and linked with the task item.
If you have the necessary permissions you can remove document links by clicking on the '-' image in the 'Documents' cell. This will not removing the document in the Shared Documents list, only the link to the task item.
If you have the necessary permissions you can add a document link by clicking on the '+' image in the 'Documents' cell. An auto-complete box will help you identify the document you wish to link with.