You very well could be blowin out gold. Your set up (motor/pump) may be to large for a 3" depending on your sluice .
Not sure of the HP of a gx200. Keene lists their 3" with a 4hp P104 pump, they list a 6.5hp P180 using a 14" x 48" box
If possible, put another sluice behind the primary box or put a pan under the discharge of the box and check it after awhile.

Try a combination of slowing the motor some and flatting out the box some.
Or a wider box.

You should not see gold below the second or third riffle.  
Gotta find the sweet spot.

Bill

Furthering Mr. Hall's suggestion, I use a LeTrap strapped off the end of my 10" sluice on my keene 2.5" highbanker when I run the larger pump. I have not seen anything get by the Le Trap. You can use any wider sluice to get this effect. Additional baffling or a stiffer/longer baffle may yield better results as well.

What type of dredge set up are you using ? Suction nozzle, jet log with/without flair, crash box or something other ?

Pics would be helpful.
Something I was thinking about Brad touches on.
At the outlet of your discharge into the box, do you have a baffle to smooth out the water as it enters the box ?
If not, employ one. A piece of rubber (mud flap). Helps smooth out the water, slows the water some, helps to settle the gold in the box.
Also, does your box have a scalper, or punch plate over the upper section of the box ? The material moves up and out of your jet, dropping onto the scalper.
This allows the small stuff drop through into a slow water area, keeping the larger rock moving off the sluice.

As for volicity, you want enough water to clear the box in about 10 -15 seconds, that may mean suction may suffer some, better to slow down and not blow gold than to hog it through loosing precious gold that one has worked hard to obtain. 

Try to keep the box discharge above and out of the water.  

Its a combination of all the above. Enough slope on the box to clear in a reasonable time, not to steep, enough water flow to maintain a reasonable nozzle suction to keep material moving through the hose and not clogging but not over powering, able to run motor/pump at something less than full throttle.

Are you running an air compressor ?

Its just that simple....

Brad, call me Bill, none of this Mr Hall stuff please.

Bill

Thanks for the tips. We'll give it a try.

Chris

Thanks for the tips. We'll give it a try.

Chris

Thanks for the tips. We'll give it a try.

Chris

Thanks for the tips. We'll give it a try.

Chris

Thanks for the tips. We'll give it a try.

Chris

Thanks for the tips. We'll give it a try.

Chris

Thanks for the tips. We'll give it a try.

Chris

Thanks for the tips. We'll give it a try.

Chris