We analyze the asymptotic performance of nonbinary spatially-coupled low-density parity-check (SC-LDPC) codes built on the general linear group over the binary erasure channel. In particular, we prove threshold saturation of the belief propagation decoding to the so called potential threshold, using the technique recently introduced by Yedla et al. We rewrite the density evolution in an equivalent form which is suited for the use of the potential function, and we define a potential function in a slightly more general form than one by Yedla et al., in order to make it applicable to the case of nonbinary LDPC codes. Moreover, the existence of the potential function is discussed for the general case, and some existence conditions are developed.