The Brillouin gain spectrum (BGS) provides key information for stimulated Brillouin scattering (SBS), such as the Brillouin frequency shift (BFS), Brillouin spontaneous linewidth, and Brillouin gain coefficient. In this study, we theoretically investigate the field distributions and BGS characterization of Ge-doped, Al-doped, and Al/Ge co-doped fibers. Additionally, we analyzed and compared the relationship between the BGS and acoustic refractive index. In particular, we demonstrate the crucial role played by acoustic modes in anti-waveguide structures. The simulation results show that the Brillouin gain coefficient decreases with a decreasing acoustic index in the fiber core region. Furthermore, we experimentally measure the SBS threshold and BGS of the Al/Ge co-doped fiber to examine the validity of the numerical model. The simulated and experimental results are consistent.