The first one comes much sooner than that
christep, you mean the first gong comes much sooner than the gear up after T/O? That would mean before the usual 50-100 ft at which the gear is retracted. You'd still be on the runway.Or maybe I didn't understand right?
why the cabin crew needs to know that I'm not sure
The gongs are signal for company procedures. It's the way the F/D tells us at which stage of the flight we are. Example, in my company the first gong will tell us we are at 10.000ft, which in our procedures means we are relased. But at different stages of flight, and with different use (2 or 3 gongs, HI or LOW chime or bot HI-LOW) mean different things (CCcalls, F/D calls, emergency calls, toilet smoke, toilet calls,etc...)
CC and F/D are synchronised during the different phases of flight, meaning that since we know eachother's procedures we will be aware more or less what's going on in the F/D (for CC) and in the cabin (for F/D). This is very important since we are working jointly on the same mission.