Like the later GAN/UMA standard, GIP makes use of a technology that doesn't require licensed spectrum to expand capacity and allow end users, in theory, to improve coverage in areas difficult to reach via large, external, cell towers.
The most popular profile for DECT is GAP, which is used to provide cordless phone service, but this is not used for GIP.
The system supports handoff, and authentication is done via the GSM SIM card as normal.
However, DECT terminals need to authenticate themselves against the base station, and this added layer is implementation dependent.
However, the industry has gone in the direction of using GAN/UMA, which substitutes an 802.11 or Bluetooth air interface for GSM/UMTS's and as such can use unmodified commodity infrastructure.