Can the ILS glideslope be intercepted from above?
Assuming that you're asking about intercepting the glideslope from above rather than the localizer, the answer is that it is definitely NOT recommended.

There are at least two significant problems if you do this. First, is the fact that at high vertical angles there can be false glideslopes. Looking at the diagram below, if you're coming at the glideslope from above and the needle starts to move, are you intercepting the real 3&deg; glideslope or the false 9 &deg; glideslope?



The diagram above is the best I could find, but it has one error. It shows a "Flag no signal" at 6&deg;. Actually what you'll have there is another false glideslope, but one which has reverse sensing.

You can, of course, do a reasonability check if you know your distance from the runway, or you could compare your vertical speed if following it with what you know your vertical speed should be, or on the 6&deg; line a command to go up when you know you should go down would be a hint, but it's unwise in my opinion to put yourself in a situation that increases your work load that much during an instrument approach.

You asked about GPWS warnings. None of the aircraft I flew that had GPWS would sound a GPWS warning just from receiving a false glide slope. However, all of them would have sounded the GPWS if you followed a false glideslope and were close to the ground because doing so would give you an excessive sink rate.

A second problem with approaching from above is that the vertical angular width of the glideslope as measured from needle full down to needle full up is very small. As I remember only about one degree. Whatever it is, though, the point is that if you descend into the glideslope with a vertical speed of only a few hundred feet per minute greater than the glideslope itself is descending, the needle is going to go from full down to full up very quickly. Without working it out, I'd guess that if you're inside the outer marker you might well miss its movement if your instrument scan had you away from the needle more than a second.

In the late 1970s a friend of mine was flying a charter in a Cessna 310. He unwisely decided to intercept the glideslope at Salem, Oregon from above. The aircraft hit the ground in the vicinity of the outer marker, killing all aboard. During the accident investigation, the NTSB found from radar data that he had a descent rate greater than 1,000 fpm going through the glideslope.