Someone here seems to take the attitude that these cautions and warnings are nothing but nuisance...I wish you wouldn't take it too slightly.
Perhaps with the exception of Mode 3, "Don't Sink" and occasionally some nuisance alerts, any of the GPWS warnings could literally mean you are seconds from being a grease spot on some hills. I wouldn't turn it off if I were you.
I was on about half a dozen of GPWS flight tests and my impression was that these warnings were darn hard to get. When we get them I'd still remember the chills that ran down my spine, thankful that we were inducing these warnings on purpose. As I recalled, to get Mode 1 "Sink Rate" and the subsequent "Whoop Whoop Pull Up", we had to start out at 2500ft AGL diving straight at the ground at about 3000fpm. It's normal to pull 2.5g in the pull up maneuver. Quite an exciting ride in retrospect. Unless you fly a Dash 8 doing 6 degree path descents at high altitude with high ground speed, if you hear this in real life, better arrest that rate quick. You'd get less than a minute to live, usually.
To test Mode 2, we aimed at a mountain, flying about 200 ft above the top. Because it's Radar Altimeter based, you'd get the "Terrain Terrain" about 15 seconds before impact, the "Whoop Whoop Pull Up" about 10 seconds before impact (if you happen to fly below the peak). You hear this, you immediately aim for the moon. The Cali, Columbia accident in 1995...the airplane got the warning 12 seconds before it hit the mountain. Yes, that was a Mode 2. The Enhanced GPWS (TAWS) would give you about 60 seconds caution (Terrain) and 30 seconds (PULLUP).
Mode 3 was easy to get, takeoff, lose about 80 ft altitude or get a negative climb rate of more than a 100fpm or so then you get it.
To get Mode 4, I remembered you have to be really close to the ground about 500 ft AGL or so before you get "Too Low Terrain", keep getting closer then you get "Too Low Gear" then about 200 ft AGL or so you'd get "Too Low Flaps". We'd pick some flat field out in the desert to do this test. All I would see would be brown earth all around us. I'd hate to run into some high voltage power lines out there.
Mode 5 "Glideslope" wasn't too bad but you have to be a whole dot below glideslope really close to the ground (about 300 ft AGL or so) before you'd get it. I'd react quickly to any of these.
If you are lucky enough that the aircraft you fly has it, use it. Don't turn it off. It could save your life some day. I read somewhere that 70% of all accidents out there are due to one form of CFIT (controlled flight into terrain) or another. A few airliners CFIT accidents happened partly because the crew didn't react to GPWS Pull Up warnings aggressively.
The cheapest GPWS out there is about $8,000 and unfortunately it's not required for part 91 unless the a/c has 6 pax seats or more. So most GA aircraft out there do not have this gem installed. Hopefully it'll get much cheaper really soon.