@Smruti Ranjan Nayak Thanks for reaching out. From the description i understand that you needed few of your api's to use self-hosted gateway while other APIs continue to use the default gateway. You can modify the gateway settings in APIM to use a self-hosted gateway for specific APIs, while using the managed gateway for the rest of the APIs or both the gateways as shown below
do let me know incase of further queries, I would be happy to assist you.