This year on Gayatri Japam day, am in a compelling situation to travel; can we do Gayatri Japam on Avani Avittam day or a couple of days later? Is there any atonement for this?

As an alternate day has not been specified for Gayatri Japam and must be performed on the stipulated day, plan your travel; accordingly, one can do the japam either early in the morning or at the travel destination on reaching there. Make some such arrangements.
Ask us again if you are completely unable to make alternate arrangements and you have missed Gayatri Japam and we will let you know what needs to be done. No specific atonement has been specified. Since it cannot be done on any other day, Gayatri Japam must be done on the mandated day.