TITLE: GCN CIRCULAR NUMBER: 28717 SUBJECT: GRB 201020A: Redshift from GTC/OSIRIS DATE: 20/10/21 10:30:41 GMT FROM: Alexander Kann at IAA-CSIC D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), M. Blazek, J. F. Agui Fernandez, C. C. Thoene (all HETH/IAA-CSIC), G. Lombardi (GRANTECAN, IAC) and A. Tejero (GRANTECAN) report: We observed the afterglow (Ambrosi et al., GCN#28696; Adachi et al., GCN#28697; Fu et al., GCN#28701; Belkin et al., GCN#28703; Pankov et al., GCN#28708; Kumar et al., GCN#28711; Paek et al., GCN#28713) of GRB 201020A (Ambrosi et al., GCN#28696; Sakamoto et al., GCN#28705; Bissaldi et al., GCN#28706) with the 10.4m GTC telescope, at Roque de los Muchachos Observatory (La Palma, Spain) equipped with OSIRIS. The observation started on 20 October 2020 at 20:08 UT (14.34 hrs after the GRB onset) and consisted of 4 x 900s with the R1000B grism, covering the wavelength range between 3700 and 7800 AA. Observations were taken under inclement conditions, with bad seeing and thick clouds passing through. The spectrum shows a clear continuum, consistent with the Lyman forest, Ly-alpha, SiII, OI, CII, SiIV, CIV, FeII, FeII*, and AlII, all at a common redshift of z = 2.903, which we identify as the redshift of the GRB.