When items are added to GridView Form component using Dynamic Model and Grid Mode is set to Select Items, rows in GridView cannot be selected using the Select link. The issue is specific to Select Items and Dynamic Model for Items.
This is a defect in GridView component.
This issue is fixed in Workflow 8.0.
To work around the issue, please use Process Variable as an item source and have a model (or section of the flow) before going into Form to initialize and populate the variable.