What are the common GAS directives for laying out data, and how do they map to C types?

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Accepted Answer

Directives both switch sections and emit data of a chosen size — .byte/.word/.long/.quad correspond to 1/2/4/8-byte values, mirroring C's char/short/int/long.
A "variable" in assembly is just a label followed by directives that emit the right number of bytes. The size directive you pick is the type:

Directive
Purpose / C analogue

.text / .data
Switch to the code / initialized-data section

.align N
Align the next item to an N-byte boundary

.globl / .extern
Export a symbol / declare one defined elsewhere

.byte
1-byte value (char)

.short / .word
2-byte value (short)

.int / .long
4-byte value (int)

.quad
8-byte value (long, pointer)

.ascii
String, no null terminator

.asciz / .string
Null-terminated string (adds the trailing \0)

.skip N / .zero N
Reserve N bytes (zero-filled)

Gotcha: .ascii "hi" emits only h,i — two bytes. If C code will treat it as a string and call strlen/printf on it, you want .asciz/.string so the terminating \0 is actually present.
Go deeper:

Using as — the GNU Assembler manual — the complete directive list — .byte/.word/.long/.quad/.ascii/.asciz and more.
 GNU Assembler (Wikipedia) — GAS overview.

Directive	Purpose / C analogue
`.text` / `.data`	Switch to the code / initialized-data section
`.align N`	Align the next item to an N-byte boundary
`.globl` / `.extern`	Export a symbol / declare one defined elsewhere
`.byte`	1-byte value (`char`)
`.short` / `.word`	2-byte value (`short`)
`.int` / `.long`	4-byte value (`int`)
`.quad`	8-byte value (`long`, pointer)
`.ascii`	String, no null terminator
`.asciz` / `.string`	Null-terminated string (adds the trailing `\0`)
`.skip N` / `.zero N`	Reserve N bytes (zero-filled)