He wouldn't react at all: GNUstep was already an established project before his death, so he already had an opportunity to react and, unsurprisingly, he didn't care much.
Apple didn't really like OpenDarwin or whatever was the name, right. But more on trademark concerns, if I'm not wrong.
GNUstep started in 1995 back when Jobs was still at NeXT before Apple bought the company. He didn't try to stop it back then.