Alright, ladies and gents, I caved and bought some higher quality tonic water.
FYI, Seagram's tonic water and Schweppes tonic water contain HFCS. Most bars get their tonic water from a soda gun, so beware.
Gin: Seagram's - $9.99
It was cheap and I am no gin connoisseur.
Tonic Water: Q - $5.99 (sale)
The bottle looks fancy. In addition to Fodmap-friendly ingredients, it also advertises having fewer calories than HFCS tonic water.