Irregular gestational sac or G.sac means, the sac in which an embryo develops in early pregnancy that has irregular in comparison to a normal gestational sac.
Gestational sacs which are regular is good for pregnancy but an irregular gestational sac is usually a cause for concern as they are predictors of many a times poor outcome or miscarriages.
If your ultrasound shows irregular gestational sacs, then it is normally followed up by further ultrasounds in 10 days or 2 weeks if everything is ok.