The BC-778-* operates on 500 kHz only. Here is some general GG information.
The Gibson Girl in US service was actually two different models that looked almost identical:
1. BC-778-*, part of SCR-578-*. Model with 500 kHz MCW. 1942 and later.
2. T-74/CRT-3. Model with 500 kHz MCW and 8280 kHz CW. 1945 and later.
The BC-778-* transmitter is a 12A6 LC-controlled 500 kHz oscillator grid-modulated by a 12SG7 1000 Hz AF oscillator/amplifier. It produced a nominal 5 watts MCW output when hand cranked at 80 to 100 rpm. Instructions say 20 seconds of cranking is required to warm indirectly-heated tube cathodes before seeing RF output.
SOS (. . . - - - . . . sent as one character) is sent six times followed by a 20 second dash.
The 20-second dash allows rescue force direction finders to determing signal direction.
The GG does NOT send the maritime international 500 kHz distress Auto-Alarm signal (12 four-second dashes separated by one second sent in a one-minute interval). Aircraft and naval ships carried no Auto Alarm receivers. It is also unlikely that a hand-cranked keyer would time the dashes with sufficient precision to trip an AA receiver's primative decode circuitry.
Proper crank speed is indicated by one light bulb and actual RF output by another. Manual keying is possible with a large pushbutton. Instructions warn the operator to send no faster than 5 wpm because of the difficulty of sending readable Morse with that pushbutton.
The whole set includes a box kite and two ballons with hydrogen generators to lift its 350 foot wire. On aircraft a 12 foot parachute was attached to the storage bag complete with a static line to pull the rip cord after the set's bag was pushed out of the aircraft.
Some WWII USAAF ferry flights of large aircraft above the Arctic Circle were intended to included the small dry-battery-powered Setchell-Carlson Model 591 200-420 kHz receiver (military nomenclature AN/CRR-1) with these transmitters to allow the distressed party to receive voice messages from rescuers. The AN/CRR-1 definitely existed. I have one with manual. I don't know if the AN/CRR-1 was actually used as intended.
Late in WWII the improved T-74/CRT-3 became available. Mechanically and electrically it is almost identical except that when automatic keying is selected the six SOS and one 20 second dash are sent for 45 seconds on LC-controlled 500 kHz MF MCW followed by 45 seconds on crystal-controlled 8280 kHz HF CW. Manual keying is not possible on 8280 kHz. After WWII the HF frequency was changed to 8364 kHz.
Mike / KK5F