Thank you for reaching out.
Based on your question above.
As per the Below link no details saying that Private application gateway is on mode preview https://azure.microsoft.com/en-ca/explore/global-infrastructure/products-by-region/?products=application-gateway
The link above describes Azure Application Gateway product as a whole which is generally available. The Private Application Gateway is a feature of Application Gateway v2 which is currently in public preview. This preview feature was announced on 04 April, 2023 and the date for general availability of the feature has not been declared yet.
Hope this clarifies your question. Please let me know if there are any additional queries. Thank you!