Grain boundary (GB) segregation can significantly impact the mechanical properties of structural materials, such as high-strength steels. Particularly in the context of hydrogen enhanced decohesion (HEDE), one of the proposed failure mechanisms, where H accumulation between cleavage planes is expected to reduce the interplanar cohesion. In the present work, spin-polarized density functional theory calculations were carried out to investigate the influence of H and C on the decohesion of the $\Sigma5(310)[001]$ and $\Sigma3(112)[1\bar{1}0]$ symmetrical tilt GBs in $\alpha$-Fe. The more open local environment at the $\Sigma5$ GB offers more energetically favorable space for segregation than the $\Sigma3$ GB, indicating a significant role of the local atomic environment in the effect of H and C segregation at the GB. In the $\Sigma5$ GB, C increases the strength, whereas H leads to a significant reduction, up to 60 \%. In contrast, in the $\Sigma3$ GB, the effect of both elements is limited. This result suggests that close-packed GBs are less susceptible to H embrittlement. Furthermore, the traction-separation behavior of the segregated GB was calculated in both thermodynamic limits of constant concentration and constant chemical potential, which could enable comparison with experimental data. An investigation of the co-segregation of H with other common alloying elements, such as C, V, Cr, Mn, and B, found that the presence of these elements can significantly impact the GB strength and counteract the detrimental effect of H. Our results provide atomistic insights into the HEDE phenomena in ferritic steel GBs, while proposing strategies to address the intricacies of the methodological aspects involved in the \textit{ab initio} study of segregated interfaces.