Depending on your requirements, there are a couple of options:
1) Use a tagged field so that each transaction stores the Gift Aid value against it
This method would mean that supporters mark donations as Gift Aided or not as they are submitted. This is the recommended way. To do this, you’ll need to create a new supporter record field (if you haven’t already) tagged as Tax Deductible and add it as a checkbox field to your donation form block.
When you export donations from this page you will see either a “Y” or an “N” in the column Campaign data 14 when exporting as transaction format. You can also see the status within the supporter’s transaction history gadget in their lookup page.
2) Use an opt-in question to store the Gift Aid value on a per supporter basis
If you wish to mark the supporter as having gift aided or not, rather than per transaction, you can use an opt in question. You can then export and filter supporters based on whether or not they opted in to Gift Aid in their last donation.
This data can be seen just like with any other opt-in question: under the supporter’s opt-in questions in Lookup Supporter, or via an export.