The Android platform gdb is built as part of the NDK build, from sources at https://android.googlesource.com/toolchain/gdb/. To build it, follow the steps at https://android.googlesource.com/platform/ndk/+/master/README.md, and run `python checkbuild.py --module gdbserver`. The current prebuilt versions are built from the sources described by the manifest xml file next to the binaries.