What the universe is made of is a fundamental question. And yet dark matter, which makes up >80% of the matter in the universe, is of unknown nature. One approach to identifying dark matter is to search for cosmic antiparticles produced when dark matter annihilates or decays. But a “smoking gun” signature is desirable, so that the dark matter generated cosmic particles are not confused with conventional cosmic rays. Low-energy antideuterons have long been known to represent such a “smoking gun”. The General Antiparticle Spectrometer (GAPS) is the only experiment optimized specifically to search for low-energy (< 0.25 GeV/n) cosmic antiprotons, antideuterons, and antihelium. Its goals are (i) to deliver a first-time detection of cosmic antideuterons, an unambiguous signal of new physics that probes a wide array of dark matter models, or to improve upon previous antideuteron limits by two orders of magnitude, (ii.) to provide a precision antiproton spectrum in a previously unexplored energy region, permitting leading constraints on light dark matter, the best limits on primordial black hole evaporation on Galactic length scales and novel constraints on cosmic-ray propagation models, and (iii.) to investigate recent AMS claims of evidence for cosmic antihelium. GAPS will execute three ultra-long duration balloon flights from Antarctica. I will review the current status of antimatter searches for dark matter, and discuss progress on building the GAPS experiment.