isoelectronic is close but doesn't cut it for me. There is a mathematical symmetry about GaAs in view of Ge and it goes like this: (31 + 33)/2 = 32 or, in chemical logic symbols: (Ga + As)/2 = Ge.
Like gallium arsenide, germanium is a photovoltaic material. Google "germanium solar cell" and you will find cutting edge research involving blends of gallium arsenide with germanium. I'm glad there is on-going research into new materials because I am not sure we should be putting arsenic on every rooftop much like we're putting mercury in every lightbulb.
A similar "bookend relation" occurs a couple rows up in the Table between boron, carbon, and nitrogen. Look how boron and nitrogen bracket carbon:
Once again, (5 + 7)/2 = 6. And just like carbon, boron nitride (BN) has both graphite- and diamond-like structures. One type of BN is even harder than diamonds: link
I see a pattern here: the centrality of the carbon group, C, Si, Ge, etc. to the family of main group elements: