The use of Glass-Fibre Reinforced Polymers (GFRP) to reinforce concrete elements has been widely accepted as an adequate strategy to provide more durable structures under severe environments such as marine structures, and in temporary or neutral electromagnetic applications. Nowadays, several codes and guidelines that provide formulation to design GFRP RC have been released or are under review, some of them with different levels of approximation. Because of the inherent novelties present in GFRP materials, different hypothesis have been considered among guidelines and codes, leading to significant differences in the resultant design assumptions, i.e. mainly in terms of GFRP bars amount. This paper presents a critical analysis of the application of six guidelines and codes (Model Code 2020, Annex R of the second generation of Eurocode 2, AFGC, CNR-DT 203, ACI 440.11, and CSA S806) in the design of the longitudinal GFRP reinforcement of a concrete beam considering verifications for both ultimate and serviceability limit states. The aim of this study is to identify and clarify the distinctive aspects of each guideline and code as well as the assumptions that may not be directly captured in the daily practice of engineers unfamiliar with designing GFRP reinforced concrete (RC) elements. The results are presented in terms of the minimum number of bars that accomplishes each of the considered limit states (i.e., flexural capacity, stresses in materials, crack width and deflections).