I'm not sure what you are talking about. BOTH PCG and GMRES are written in MATLAB itself. So the source code is provided to you. All you had to do was look.
Should you modify the code? NO! At least, if you do, then make a copy of it, with a new name. Do NOT modify the code and leave the name unchanged.
Can you provide your own solver? READ THE HELP! Here is the pertinent part:
X = PCG(A,B,TOL,MAXIT,M) and X = PCG(A,B,TOL,MAXIT,M1,M2) use the
% symmetric positive definite preconditioner M or M = M1*M2 and
% effectively solve the symmetric system inv(H)*A*inv(H')*Y = inv(H)*B
% for Y, where Y = H'*X and M = H*H'. The algorithm does not form H
% explicitly. If M is [] then a preconditioner is not applied. M may be a
% function handle MFUN returning M\X.
So you can provide a function handle that does the work.
