The idea of space-time coding, devised for multiple-antenna systems, can be applied to communication over a wireless relay network. This paper is on the design of practical distributed space-time codes for wireless relay networks, in which uses amplify-and-forward (AF) scheme in a way that each relay transmits a scaled version of linear combination of the received symbols. We use the recently presented GABBA codes, which are systematically constructed, orthogonally decodable, full-rate, full-diversity space-time block codes, in a distributed fashion. Our scheme is generalized to any number of relays with linear orthogonal decoding in the destination, which make it feasible to employ large number of potential relays to improve diversity order. Assuming BPSK modulation and maximum likelihood (ML) detection, we derive a formula for the bit error probability of the investigated scheme in Rayleigh fading channels. Our analytical results have been confirmed by simulation results, using full-rate, full-diversity distributed codes.