  We show a hard induced superconducting gap in a Ge-Si nanowire Josephson
transistor up to in-plane magnetic fields of $250$ mT, an important step
towards creating and detecting Majorana zero modes in this system. A hard
induced gap requires a highly homogeneous tunneling heterointerface between the
superconducting contacts and the semiconducting nanowire. This is realized by
annealing devices at $180$ $^\circ$C during which aluminium inter-diffuses and
replaces the germanium in a section of the nanowire. Next to Al, we find a
superconductor with lower critical temperature ($T_\mathrm{C}=0.9$ K) and a
higher critical field ($B_\mathrm{C}=0.9-1.2$ T). We can therefore selectively
switch either superconductor to the normal state by tuning the temperature and
the magnetic field and observe that the additional superconductor induces a
proximity supercurrent in the semiconducting part of the nanowire even when the
Al is in the normal state. In another device where the diffusion of Al rendered
the nanowire completely metallic, a superconductor with a much higher critical
temperature ($T_\mathrm{C}=2.9$ K) and critical field ($B_\mathrm{C}=3.4$ T) is
found. The small size of diffusion-induced superconductors inside nanowires may
be of special interest for applications requiring high magnetic fields in
arbitrary direction.
