Gerrard's triggered ability returns artifact and creature cards even if they were put into your graveyard while Gerrard wasn't on the battlefield, so long as they were put there this turn.

Permanent spells that were countered earlier in the turn never entered the battlefield, so they won't be returned by Gerrard's ability.

Gerrard's triggered ability returns artifact and creature cards even if Gerrard leaves your graveyard before you can exile it.

If Gerrard is your commander and you can exile it from your graveyard as its triggered ability resolves, you'll still be able to put it into the command zone after you return your artifact and creature cards to the battlefield.