The best you can do is an alias in your shell:

alias cdgp='function _cdgp(){ cd $1; git pull --autostash; };_cdgp'

Then do:

cdgp ~/mygitrepo

You could change the behavior of cd

cd() {
  command cd "$@"
  [[ -d ".git" ]] && git pull --autostash
}

I think it's good to use test there so that there won't be an error every time you cd into a directory that isn't a git clone.

I have something similar in place for activating virtualenv every time I enter a directory that has a virtualenv setup with the same name.