The link gain for a directly modulated microwave-photonic link based on a transistor-laser (TL) is derived by making use of signal flow graph techniques. In contrast to conventional laser diodes, the three-terminal TL introduces an extra microwave port for impedance tuning of link parameters. Gain contour simulations using a small-signal equivalent circuit model for the TL indicate the potential for increased link gain compared with links that are terminated in 50 Omega at all ports.