Longer is not better in this case. GMRS is UHF. A quarter wavelength is about 6 inches. The NA-701c is a dual band antenna designed for commercial bands, and GMRS. It will work, but you really do not need a dual-band antenna. The NA-771 is a dual ham band antenna, Being longer, It will do better on VHF 2-meter ham band, but not as good on GMRS, which is out of its operating range.
If you want greater range, use an external antenna on your roof, or on the top of your vehicle. For portable operation, you aren't going to get much more range with any rubber-duck antenna - the factory one will do just fine.
