GAS is the GNU Assembler, that GCC relies upon.
Find it at the same place where you've found GCC, in the binutils package. The latest version of binutils is available from http://sources.redhat.com/binutils/.
Because GAS was invented to support a 32-bit unix compiler, it uses standard AT&T syntax, which resembles a lot the syntax for standard m68k assemblers, and is standard in the UNIX world. This syntax is neither worse, nor better than the Intel syntax. It's just different. When you get used to it, you find it much more regular than the Intel syntax, though a bit boring.
Here are the major caveats about GAS syntax:
Register names are prefixed with %, so that registers are %eax, %dl and so on, instead of just eax, dl, etc. This makes it possible to include external C symbols directly in assembly source, without any risk of confusion, or any need for ugly underscore prefixes.
The order of operands is source(s) first, and destination last, as opposed to the Intel convention of destination first and sources last. Hence, what in Intel syntax is mov eax,edx (move contents of register edx into register eax) will be in GAS syntax mov %edx,%eax.
The operand size is specified as a suffix to the instruction name. The suffix is b for (8-bit) byte, w for (16-bit) word, and l for (32-bit) long. For instance, the correct syntax for the above instruction would have been movl %edx,%eax. However, gas does not require strict AT&T syntax, so the suffix is optional when size can be guessed from register operands, and else defaults to 32-bit (with a warning).
Immediate operands are marked with a $ prefix, as in addl $5,%eax (add immediate long value 5 to register %eax).
Missing operand prefix indicates that it is memory-contents; hence movl $foo,%eax puts the address of variable foo into register %eax, but movl foo,%eax puts the contents of variable foo into register %eax.
Indexing or indirection is done by enclosing the index register or indirection memory cell address in parentheses, as in testb $0x80,17(%ebp) (test the high bit of the byte value at offset 17 from the cell pointed to by %ebp).
Note: There are few programs which may help you to convert source code between AT&T and Intel assembler syntaxes; some of the are capable of performing conversion in both directions.
GAS has comprehensive documentation in TeXinfo format, which comes at least with the source distribution. Browse extracted .info pages with Emacs or whatever. There used to be a file named gas.doc or as.doc around the GAS source package, but it was merged into the TeXinfo docs. Of course, in case of doubt, the ultimate documentation is the sources themselves! A section that will particularly interest you is Machine Dependencies::i386-Dependent::
Again, the sources for Linux (the OS kernel) come in as excellent examples; see under linux/arch/i386/ the following files: kernel/*.S, boot/compressed/*.S, math-emu/*.S.
If you are writing kind of a language, a thread package, etc., you might as well see how other languages ( OCaml, Gforth, etc.), or thread packages (QuickThreads, MIT pthreads, LinuxThreads, etc), or whatever else do it.
Finally, just compiling a C program to assembly might show you the syntax for the kind of instructions you want. See section Do you need assembly? above.
Good news are that starting from binutils 2.10 release, GAS supports Intel syntax too. It can be triggered with .intel_syntax directive. Unfortunately this mode is not documented (yet?) in the official binutils manual, so if you want to use it, try to examine http://www.lxhp.in-berlin.de/lhpas86.html, which is an extract from AMD 64bit port of binutils 2.11.
Binutils (220.127.116.11.25+) now fully support 16-bit mode (registers and addressing) on i386 PCs. Use .code16 and .code32 to switch between assembly modes.
Also, a neat trick used by several people (including the oskit authors) is to force GCC to produce code for 16-bit real mode, using an inline assembly statement asm(".code16\n"). GCC will still emit only 32-bit addressing modes, but GAS will insert proper 32-bit prefixes for them.
GAS has some macro capability included, as detailed in the texinfo docs. Moreover, while GCC recognizes .s files as raw assembly to send to GAS, it also recognizes .S files as files to pipe through CPP before feeding them to GAS. Again and again, see Linux sources for examples.
GAS also has GASP (GAS Preprocessor), which adds all the usual macroassembly tricks to GAS. GASP comes together with GAS in the GNU binutils archive. It works as a filter, like CPP and M4. I have no idea on details, but it comes with its own texinfo documentation, which you would like to browse (info gasp), print, grok. GAS with GASP looks like a regular macro-assembler to me.