> You need a Mac to properly learn it. gcc+GNUStep don't really count.
Out of curiosity, why not? You don't get the Cocoa/Cocoa Touch APIs, but those frameworks are independent of the language itself. Relying on them is akin to saying you can't properly learn C++ without Qt, or Ruby without Ruby on Rails.
Cocoa and Cocoa Touch are integral to being paid for Obj-C, and they're by far the hardest part of it.
Learning objective-c, especially if you already know C, is pretty easy. The syntax is a bit weird but not hard, then you've got a few quirks and stuff, but fundamentally you can build a working model of the language in half a day tops if you're already a developer.
Understanding Apple's conventions (libraries, memory management, etc...) and merely knowing where to find the stuff you need in the doc, that's what take time. Cocoa is huge and is a complex beast, even if you don't dive into the C "Core*" stuff and remain at the NS/UI levels.
Out of curiosity, why not? You don't get the Cocoa/Cocoa Touch APIs, but those frameworks are independent of the language itself. Relying on them is akin to saying you can't properly learn C++ without Qt, or Ruby without Ruby on Rails.