Set the wall clock time or change GMT offset without ntp server?
The functions configTime and getLocalTime are from ESP32 core file esp32-hal-time.c. This wraps the ESP32 SDK functions. You can use the SDK functions to set the time and zone. They work with the RTC peripheral of the ESP32.

You can use setTimeZone to set the time zone.

The time.h is standard C time library. The ESP32 SDK makes it to work with the built in RTC (reference). To set the time it gets complicated. You would have to use undocumented SDK time functions.