Yes, but it MUST also control the power to the VRTC pin (VBAT on GS1011M). The Gainspan chips are architected under the assumption that the RTC is able to control turning on and off power to the remainder of the chip, using the DC_DC_CNTL signal. When the RTC is going to change the power status of the core logic, it first 1) isolates itself from the core logic to prevent RTC register corruption, and 2) asserts reset to the core logic. When power is removed without the RTC's knowledge, neither of these steps occurs. Therefore RTC registers might get corrupted, and the core logic might not start up correctly because reset was not asserted.
If the customer wants to retain RTC data while power is off, then he must use "Host Commanded" mode, where the dc_dc_cntl signal controls turning power on/off. The host then issues aT+PSSTBY command to turn power off, and toggles an RTC_IO (alarm) pin to wake the module back up.