Back to the original question...
if you can find a counter that has enough bits, and a clock to drive it at some (Nyquist) multiple of your target measurement frequency, you can 'start' or reset the counter at the first trigger point, and stop it at the second trigger event.
The count value is the number of clock cycles you're measuring.
This clock-counter will be a lot cheaper, and easier to find than your multi-GHz pretty censor.