The largest technological application of GMR is in the data storage industry. IBM was first to put on the market hard disks based on GMR technology, and nowadays all disk drives make use of this technology.
The GMR read head sensor in a hard disk is built using a spin valve. Spin valve resistance demonstrates a steep change in the small field range close to H=0. As the magnetic bits on the hard drive pass under the read head, the magnetic alignment of the sensing layer in the spin valve changes resulting in the resistance change.
Other applications of GMR are as diverse as automotive sensors, solid-state compasses and non-volatile magnetic memories.