This link (passed to us by the Friends of Geilston, but run by the National Trust for Scotland) gives 2 offers of around a half price entry ticket to the gardens until the end of October. A maximum of 3 tickets can be bought per person and they can even be used at special events such as the forthcoming Apple Day on Saturday October 5th.
Each ticket is valid for a week and the offers are either for 2 adults, or for a family – of up to 2 adults and 4 children.
https://www.itison.com/Glasgow/deals/tickets-to-geilston-garden-west-dunbartonshire