If you want to proceed for experimental or hobbyist purposes, you must find the exact archived version that used to support Gingerbread. The final builds supporting Android 2.3.6 are generally within the series or older. Step-by-Step Download Guide

For many, the Gingerbread era was defined by iconic hardware like the Samsung Galaxy S II or the original Motorola Droid. These devices were built to last, and seeing them sit in a drawer often sparks the desire to repurpose them as simple communication tools. In theory, installing an older APK (Android Package) should allow the phone to function. Users often search for versions around the 2.16.x or 2.18.x range, which were among the last to officially support the aging operating system. The Wall of Forced Updates

WhatsApp code includes a built-in "time bomb" or expiration date. Once that date passes, the app refuses to connect to the network and forces an update.

Features like voice/video calls, GIFs, and modern sticker packs will not work.

Download WhatsApp Old Version for Android 2.3.6: A Complete Guide

Furthermore, WhatsApp’s latest codebase (v2.22 and above) uses Android APIs (Application Programming Interfaces) that simply do not exist in Gingerbread. Trying to sideload a modern APK will result in a "Parse Error" or an immediate crash.