None of those devices can be used as computer monitors except the Gateway monitor.
One thing to keep in mind when it comes to displays is
the actual resolution and not necessarily size. A vast
majority of monitors out there in the low to mid price range
have a resolution of 1920x1080. Reason, this is a standard
So, 21.5", 24", 27" will have the same number of pixels, and the
larger screen sizes will have bigger pixels. So, you won't get any
more desktop on the screen with a 27" than you will with a 21.5".
The reason the Apple display and others like it are so expensive
because they pack more pixels into the display (33% more in
both vertical and horizontal). For example on Amazon (all 27", 2560x1440):
NEC PA271w-bk $952
Dell Ultrasharp U2711 $785 (on sale reg. $949)
HP ZR2740w $706 (also on sale reg. $860)
to mention a few.
The result of the higher resolution is it can pack a little more desktop
on the screen (really helpful if you work with schematics or architectural
drawings) and images and such will appear sharper.
Just something to keep in mind when making your final decision
that size isn't everything.