This one needs a name. Somebody must have done it before. It's the Gin version of a Guardian Cocktail of the week, originally with Irish Whiskey where it's called an "Irish Maid". Great for summer evenings.
15ml St Germain
15ml Lemon Juice
7ml double syrup (or 15ml single)
Three slant cut cucumber slices
Put a rocks glass in the freezer with some rocks.
Muddle 2 slices of cucumber. Add the liquids and ice. Shake hard. Double strain into the rocks glass. Garnish with the 3rd cucumber slice and/or a lemon twist.
Gin sours take some care to get the dilution, and sweet-sour-alcohol balance right. You may need several goes to find the sweet spot.