GEMS is an IRAM 30 m Large Program whose aim is determining the elemental depletions and the ionization fraction in a set of prototypical star-forming regions. This paper presents the first results from the prototypical dark cloud Taurus molecular cloud (TMC) 1. Extensive millimeter observations have been carried out with the IRAM 30 m telescope (3 and 2 mm) and the 40 m Yebes telescope (1.3 cm and 7 mm) to determine the fractional abundances of CO, HCO ⁺ , HCN, CS, SO, HCS ⁺ , and N ₂ H ⁺ in three cuts which intersect the dense filament at the well-known positions TMC 1-CP, TMC 1-NH3, and TMC 1-C, covering a visual extinction range from A _V ~ 3 to ~20 mag. Two phases with differentiated chemistry can be distinguished: (i) the translucent envelope with molecular hydrogen densities of 1-5 × 10 ³ cm ^-3 ; and (ii) the dense phase, located at A _V > 10 mag, with molecular hydrogen densities >10 ⁴ cm ^-3 . Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C ⁺ /C/CO transition zone (A _V ~ 3 mag) where C/H ~ 8 × 10 ^-5 and C/O ~ 0.8-1, until the beginning of the dense phase at A _V ~ 10 mag. This is consistent with the grain temperatures being below the CO evaporation temperature in this region. In the case of sulfur, a strong depletion should occur before the translucent phase where we estimate an S/ H ~ (0.4-2.2) × 10 ^-6 , an abundance ~7-40 times lower than the solar value. A second strong depletion must be present during the formation of the thick icy mantles to achieve the values of S/H measured in the dense cold cores (S H ~ 8 × 10 ^-8 ). Based on our chemical modeling, we constrain the value of ζ _{H 2} to ~(0.5-1.8) × 10 ^-16 s ^-1 in the translucent cloud.