The gate time on a counter indicates the time period (generated by the internal reference clock) used for counting, e.g., if the gate time is 0.01 second, and 1MHz is applied, 10,000 cycles will accumulate during the gate time. This means that the resolution is 100Hz. The decimal location is shifted over two places in order to show 1MHz. If you change the gate time to 1 second, the resolution will be 1Hz, but your counter will only update once per second. Looking at **broken link removed**, it appears that low frequencies are measured by counting the internal clock, gating it with the input signal (i.e., measuring the input period), and taking the reciprocal. This is the only way I can understand how they get 100nHz resolution on a 1Hz signal.
You will find links to the datasheet and the user manual on this page also. The user manual has some discussion of gate time, but never seems to define it.