A global secondary index supplies another partition and optional sort key for a different access pattern. It adds write and storage work, so it isn't a free substitute for knowing how the application queries data. An item without the index key attributes may not appear in that index. A GSI uses eventually consistent reads, which matters immediately after an update.
Capacity mode changes how request capacity is managed. On-demand can fit uncertain traffic, while provisioned capacity fits a planned throughput model with appropriate scaling. Neither mode fixes a poor key that concentrates traffic on one value. In these tasks, separate the access pattern, index population, consistency requirement, and capacity decision. A table-wide scan is still a scan after you select a different billing mode.