Gear cable outer is such for a reason, compression as mentioned above. It also resists being bent through too tight angle and thereby prevents a false shift when you turn.
Brake cable outer is much more flexible and can be bent much tighter as there is more 'tolerance' in a brake set up.
You could run gear outer for brake cable, just check if the cable runs impair the turning of the bars. Also gear cable outer rips the shit out of your frame, I'd not use it for that reason alone! Brake outer is much softer and kinder.
Somerset, like 'Heartbeat' with newer cars!
Retro MTB: 1992 Cannondale M1000 Deore XT M730. 1992 Raleigh Dyna Tech Quantum Deore XT.
Retro Road: 1992 Eddy Merckx Corsa Extra Telekom Shimano 600 STI. 1993 Dyna Tech Ti Campagnolo Athena 8 Ergopower.
Modern: 2004 Wilier scandium Ultegra 6600. 2014 Wilier GTR Ultegra 6800.