Hi,
Just updating this 6 year old bug report. The last comment says it's not possible to ignore environment when using the C-API. I don't know back then, but today it is possible. I have just built gdb 8.2 against python 3.7.1, and patched gdb with this simple patch, which apparently works:
--- gdb/python/python.c
+++ gdb/python/python.c
@@ -1726,6 +1726,9 @@
#endif
#endif
+ // Force using the toolchain python without being troubled by $PYTHONHOME or $PYTHONPATH.
+ Py_IgnoreEnvironmentFlag = 1;
+
Py_Initialize ();
PyEval_InitThreads ();
Cheers,
Romain