When uploading a git commit for code review on Gerrit (2.8), I would like to be able to set the topic field (e.g. g-tune-pme-reform here https://gerrit.gromacs.org/#/c/3453/3). How can this be done from the command line? Can it be done in the original git push?
yes, it is possible using the push command:
git push origin HEAD:refs/for/master/<topic_name>