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Umm... this could be stupid, but here it goes:
I've already managed to get the bootstrap and the loader working, and I'm writing the kernel in C now. I loaded the IDT and a basic GDT with 3 descriptors (a null descriptor, a code desc and a data/stack code - I' m using the Flat Mode) in the loader(asm), and I have a gdt pointer in the kernel code (the address of this pointer is obtained during compilation with another program) that looks like this: My goal is to develop Hardware Task Switching, so I also created a TSS struct. I'm thinking in just 3 task right now. This means something like: Ok, now I want to add a new descriptor for each tss, and my question is... How do I get the linear address of the begining of each Tss? That's it, I think that I can't just write: right? (Of course, the tssdesc struct is not like I 've shown, that was just a simple example). I mean, &mytss[ 0 ] isn 't just an offset? How do I get the linear address under Protected mode? (I have switched to PM in the Loader, asm code)
(Sorry for my bad english, and if the question is too stupid - I'm new at this)

The unary & operative gives you the address of an object. The fact that you're in protected mode doesn't alter this any. Your example looks fine to me (save for the fact that for forum messed up the formatting a bit). As another example:

((tssdesc *)_gdt)[4]->base = &mytss[1];

Nothing wrong with that...