Nigeria striker Olanrewaju Kayode made his sixth straight appearance for Girona, coming on in the 60th minute for Michael Olunga.
Two own goals, first by Aday in the 17th minute and then Gorka Iraizoz Moreno in the 48th minute and a 69th minute goal from Luis Suarez, made it six wins out of six for Barcelona so far this season.
The win means Barcelona maintain top spot in the league table on 18 points, four points ahead of second-place Atletico Madrid.
LaLiga champions, Real Madrid, bounced back from their surprise 1-0 midweek loss to Real Betis as they edged Alaves 2-1 thanks to a Daniel Ceballos brace.
Madrid are now fourth on 11 points in the league standings.