There's really no vice versa - you can combine GPL code with AGPL code, but while both parts remain under their respective license, the resulting combination as

There's really no vice versa - you can combine GPL code with AGPL code, but while both parts remain under their respective license, the resulting combination as a whole then has to follow the AGPL rules ("The terms of this License will continue to apply to the part which is the covered work, but the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such."), so as soon as you incorporate even a tiny bit of AGPL code in your mostly-GPL project, users have to treat it as an AGPL project.