The operations of use start around line 909.  Here's a link to that line: https://dev.gnupg.org/source/libgcrypt/browse/master/cipher/rsa.c$909

This source can also be cloned at https://dev.gnupg.org/source/libgcrypt.git .  I've cloned it locally.

The basic operations of use are public() and secret().  They basically just call out to mpi_powm, which is a wrapper for gcry_mpi_powm().  There's a #define in gcrypt.h, which is a different file generated from gcrypt.h.in .

Before moving off rsa.c, it's notable that:

- public() basically just wraps mpi_powm, using the same structures

- secret() has an additional step to remove leading zeros

- secret() has a special form that might be used when p and q are known called secret_core_crt()

- there's something else called secret_blinded() that is likely documented in a header file or elsewhere

Basically, everything of interest is likely in mpi_powm.

I'm taking it slower now, because complexity will increase as we get deeper.  mpi_powm could look frighteningly new.