The original antenna is 230mm long and performs really well. It out performs the stock Baofeng antennas previously supplied with their radios by miles on RX and TX. It has an SMA female type connector for the GT-3, GT3-MKII and GT3-TP MKIII series of radios.
The new INF-661 is 360mm in length, almost the same as the Nagoya NA-771. It performs a little bit better than the INF-641 but nothing to shout home about. I tested it briefly with receiving my local repeater and it is better. At £4.99 they're inexpensive and will surely come in handy at some point when out and about with the GT-3.
Thanks for reading!
73's, Lewis M3HHY.