I have actually tested a Whites GMT against a GM24K side by side on the same targets in highly variable gold bearing ground areas, and frankly was not that impressed with the 24K, I will give you the good and bad.
The Good -- The 24K is much quieter as far as background Chatter, it is a little lighter and just as well balanced, it does not have the knobs that are easy to accidentally hit and change settings maybe without noticing, it is faster reacting in some ways like ground balancing.
The Bad -- The 24k has a new type of Ground Balance that is quieter and faster as I stated, BUT it seems to require some coil movement in Auto Mode, I say that because if you set it on the ground at any angle for a short time it will often Not acknowledge a scoop of soil swept near it, like when your pin pointing and looking for that target in your scoop, Annoying, and testing side by side on the same very small lead fragments I could see no increase in sensitivity over the GMT, and I think the Iron I.D was a little better on the GMT because of the Pull Trigger to Analise Function that the 24K lacks, I also think the Magnetic Sand Readings worked better on the GMT and having both readouts on the screen simultaneously Much Better on the GMT. Bottom Line -- for hunting in harsh ground they are close, but the GMT wins on Analysis even with a slower processor and noisier threshold, and if you learn to use the Black Sand Readings along with the ground readings and Probability Readout the GMT is hard to beat.
AS A SIDE NOTE -- I have recently tested the GMT side by side in the same area and again with very small lead fragments Against a Minelab Equinox 800, and to my amazement the Nox 800 was Very Close to the GMT on sensitivity to VERY small targets, and this was with the stock 11 inch coil VS the 6x10 GMT coil, it was also noticeably Much Faster with target I.D and auto ground balancing, now the I.D of course is not Probability of iron with the Nox but a range of numbers, the more negative the number the more chance it is iron, But just as accurate on iron and more so on non iron targets, but all three detectors would still be inaccurate on Large Iron such as tin can lids Etc.
If the Nox had a Magnetic Sand readout and offered a 6x10 elliptical coil I think it would beat both gold machines, the 5 in coil helps to get in tight spaces but is not that much more sensitive given the large loss of area per sweep.