What effect does switching frequency have on DC-DC boost converter voltage gain?
Almost all boost converters have a feedback loop to control the output voltage.
This means that there is no voltage gain, the output voltage is independent of the input voltage (as long as it is withing practical limits).

Perhaps you are thinking of this type of circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

The voltage gain of this circuit without any load is infinite !
Why is that so ? When S1 is closed the electrical energy from the battery charges the inductor L1. Yes an inductor can be charged with current !
This build up a magnetic field in the inductor. When S1 opens the inductor wants to keep the current flowing. So the current flows through D1 into C1.
Since L1 behaves as a current source when it is charged but S1 is open, the voltage at the anode of D1 will simply reach the value that is required to make the current flow. So with ideal components the voltage will reach an infinite value.

Switching frequency has nothing to do with this, a lower switching frequency means that S1 opens and closes faster meaning L1 is charged less per open-close cycle.