How do different metal detector frequencies help to locate items like gold in the ground?
A quote from the linked page says this: -


  Designed primarily for gold prospecting


And this raised my eyebrow straight away. See the picture of the search head: -



Anyone who has prospected for gold in the pay dirt will not want an open frame search head. What happens is that when you get a signal (hopefully gold), you move your search head to one side and, with a small trowel, start digging and putting a trowel's worth of pay-dirt on top of the search head, discarding each trowel-worth of dirt when it doesn't register a reading. Thus an open-frame search head is not that good for doing prospecting.

So, my initial thought that this was marketing BS is somewhat reinforced. 

Why are low frequencies favoured and what problems might you get with higher frequencies? A bit of a back-story: -

Traditional metal detectors use "discrimination" to select between ferrous and non-ferrous materials. All that discrimination does is shift the neutral inductive balance so that ferrous materials produce a smaller signal (up to a certain object size) whereas non-ferrous metals instantly produces a bigger signal. This method has been used for years but, since technology in metal detectors in the food industry (for instance) have measured a signal by its phase angle and amplitude (a much more complex method), the treasure hunting metal detectors are playing catch-up and may indeed be able to properly discriminate between ground effects and small pieces of metal when operating at much higher frequencies. 

Ground effects are a problem but generally, the effects produce phase angles indicating that the signal is purely resistive i.e. the ground takes energy and converts it to small amounts of heat. Good-conducting metals produce an "inductive" effect that registers a phase angle of around plus or minus 90 degrees. As you go higher in frequency the ground effect phase angle spreads out and starts to encroach on the phase angle produced by metals.

So it wouldn't surprise me that if decent tech is used you can push the operating frequency higher. Pushing the operating frequency higher means a higher induced eddy current in the "gold" and a better disturbance in the oscillation field. This basically boils down to better signal to noise ratio and hence smaller particles can be detected.

Will it detect gold rather than silver or copper or aluminium? No, look at this electrical resistivity table: -



For a given sample size/shape it will be very difficult for a metal detector to distinguish between the top four conducting metals.