Is there a way to run a report which retrieves all the purchase orders created using guided buying?

Yes, please follow the below steps:

1. Click Create > Analytical Report
2. In Source Data page, select Purchase Order fact
3. Go to Pivot Layout tab and include PO Id, Order ID, Originating System fields to detail fields
4. In Refine Data page, click Customize this form and add Originating System as a filter
5. Search for Ariba Guided Buying and include that to the report filter
6. Run the report

Now your report will only include orders that are created via guided buying.

Guided Buying > Guided Buying Requisitioning