The analysis of wavelength data of the Gd I spectrum has resulted in the detection of 201 odd and 173 even levels in addition to those reported by previous investigators. The total number of classified lines is now 5 300, belonging to 5 600 transitions between 236 odd and 371 even levels. The wavelength list including the Gd II lines contains over 18 500 lines in the wavelength region 2 468-8 752 Å. The Zeeman effect has been investigated in the wavelength region 2 700-11400 A. Of about 1 500 lines belonging to Gd I transitions the splitting has been observed. The g-values of nearly all levels could be determined. The odd terms belong to the electron configurations 4 f7 5 d 6 s2, 4 f7 5 d2 6 s, 4 f7 5 d 6 s 7 s and the newly established ones 4 f7 5 d3, 4 f7 6 s 6 p2, 4 f7 5 d 2 7 s and 4 f8 6 s 6 p ; the even terms have been associated with 4 f7 6 s2 6 p, 4 f7 5 d 6 s 6 p, 4 f 7 5 d2 6 p and the newly detected configurations 4 f 8 6 s2, 4 f8 5 d 6 s and 4 f8 6 s 7 s. Calculations in the odd configurations 4 f7(5 d + 6 s) 3 and 4 f7 5 d 6 s 7 s are given. The values of the electrostatic and spin-orbit interaction parameters give a good least-squares fit to the experimental levels.