Hello Sravan,
I have attached an archive doc on configuring Gateway for DR. Please note this doc is 6 years old and is written during the release period of Layer 7 Gateway version 5.2. While the information in it is dated, it can be considered a good working model for the product and carefully reviewed for accuracy. A new updated version is in the works today.
With regard to the OTK DB, it would be wise to have a separate DB source that the DR Gateway has an existing JDBC connection to. This is assumed that the production Gateway is housing the OTK DB and going offline for the DR test.
As for API Portal, Services would be employed by our customers for a robust DR strategy. The out of the box functionality currently exists in the portal is limited to the content level for migration between portals.
You can use the apply patch functionality discussed in the documentation.
Migrate Content Between API Portals - CA API Developer Portal - 3.5 - CA Technologies Documentation
Hope this helps