Tonic has so little quinine in it that I would have to drink several six packs to get the dose available in one capsule. Straight tonic water is not pleasant and that much gin would not be pleasant either.
Put a vacuum pump on that tonic water and evaporate the water content from the batch until it is sufficiently stout to your liking. Use a strong stainless flask for the low pressure working vessel to avoid implosion. Might need supplemental heat to keep the tonic from freezing in the flask!