No problem,
It makes about two kilner bottles, so about 500ml each. I use Erythritol as it's like for like quantities against sugar. It is syrupy which is why I serve it over lots of ice (but I've never liked fizzy drinks so I wouldn't use tonic anyway)
Method
Wash plums and pierce all over (like a jacket potato) with a sharp knife to allow the flavour to infuse
Put plums in a large, or two medium kilners (or similar), add sugar and gin. Shake, making sure jar is well sealed
.
Store in dark, cool place removing to shake every few days until Erythritol dissolves fully.
Store for three months, strain through very fine sieve, decant into bottles of choice.
I used the gin soaked plums to make a coulis. My gin this year is a fantastic dark purple due to the lovely dark skins.