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 N₂H⁺ 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 × 10³ cm⁻³; and (ii) the dense phase, located at A_V > 10 mag, with molecular hydrogen densities >10⁴ 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_H2 to ~(0.5–1.8) × 10⁻¹⁶ s⁻¹ in the translucent cloud.