SN74LVC1G123 Monostable Multivibrator IC
The term "gated" simply means that their are two inputs, \$\small \overline{\text{A}}\$ and B, only one of which can be used at a given time -- one input acts as a enable (gate) for the other.  If B is held high, then the '123 one-shot is triggered by a falling edge pulse on \$\small \overline{\text{A}}\$.  If \$\small \overline{\text{A}}\$ is held low, then the '123 one-shot is triggered by a rising edge pulse on B.

In either case, the reset line \$\small \overline{\text{CLR}}\$ must be held high.  If it goes low, then any output timing is prematurely reset.

While output timing is active, if the input \$\small \overline{\text{A}}\$ or B is triggered again, then the output is extended for a full-time period from that point, which is what is meant by being retriggerable.