He doesn't need a 'set amount' of Gaviscon during the day, it isn't like a drug where a certain dosage is needed. Gaviscon is just a thickener to make the milk stay down and you have to make it up as directed on the box to ensure the concentration of thickener to milk is correct.
Infants can only have 6 sachets a day of Gaviscon I believe and I would imagine your baby is having more than that if he's having one sachet to every 60ml bottle? Or are you not adding one to each bottle?
I know milk going to waste is a pain but you shouldn't be adding full sachets to less volume than the box instructs and I'm surprised your GP told you to - especially if it means your DS is having more than 6 sachets a day.
Could you go back to adding one sachet to 120mls and then giving 60mls of that for the feed and keep the other 60mls in the fridge ready for the next feed?
I think it's ok to keep feeds containing Gaviscon in the fridge ready for later feeds but do check. If however it can't be done it may well just mean that unfortunately a lot of milk is going to have to go to waste 
The concentration of Gaviscon to the milk should always be done correctly because if there isn't enough formula (4oz or whatever the box says) to absorb a full sachet then the milk will go very thick, start to separate etc and I imagine produce a choking hazard to the infant. And like I said earlier, if the milk is too thick the baby can't physically suck it through the teat holes either so may not feed, get angry, frustrated, seem to tire during the feeds etc.
How old is your baby and what's his normal feeding pattern in terms of frequency and volumes of bottles?