Voltage Controlled Oscillators
The formula you pasted is correct and makes perfect sense. It's purely descriptive. This gain formula is a simple way of describing the VCO by its voltage input and frequency ranges. 

A VCO operating within the MHz range, powered by 10V will have a huge gain. For every volt on its input, the output will rise by millions of Hertz. Unless you of course design a high frequency VCO with a very fine-tuned output (e.g. from 100MHz to 100.01MHz).