create and then destroy a ghash and you don't get all the memory back.  Here's a
test program:

#include <glib.h>
int main(void) {
    GHashTable *table = g_hash_table_new(g_str_hash, g_str_equal);
    g_hash_table_destroy(table);
    return 0;
}

Compile it:

localhost:~$ gcc $(pkg-config --libs --cflags glib-2.0) -Wall -Werror -o ghash
ghash.c
localhost:~$ 

And run it under valgrind:
localhost:~$ sudo valgrind --leak-check=full --show-reachable=yes
~/ghash==31032== Memcheck, a memory error detector.
==31032== Copyright (C) 2002-2006, and GNU GPL'd, by Julian Seward et al.
==31032== Using LibVEX rev 1606, a library for dynamic binary translation.
==31032== Copyright (C) 2004-2006, and GNU GPL'd, by OpenWorks LLP.
==31032== Using valgrind-3.2.0, a dynamic binary instrumentation framework.
==31032== Copyright (C) 2000-2006, and GNU GPL'd, by Julian Seward et al.
==31032== For more details, rerun with: -v
==31032== 
==31032== 
==31032== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 682 from 1)
==31032== malloc/free: in use at exit: 5,580 bytes in 8 blocks.
==31032== malloc/free: 9 allocs, 1 frees, 5,624 bytes allocated.
==31032== For counts of detected errors, rerun with: -v
==31032== searching for pointers to 8 not-freed blocks.
==31032== checked 75,996 bytes.
==31032== 
==31032== 2,016 bytes in 4 blocks are still reachable in loss record 1 of 2
==31032==    at 0x401F5C6: memalign (vg_replace_malloc.c:332)
==31032==    by 0x401F620: posix_memalign (vg_replace_malloc.c:386)
==31032==    by 0x4075B63: slab_allocator_alloc_chunk (gslice.c:1065)
==31032==    by 0x4076597: g_slice_alloc (gslice.c:614)
==31032==    by 0x4052B18: g_hash_table_new_full (ghash.c:139)
==31032==    by 0x4052B97: g_hash_table_new (ghash.c:110)
==31032==    by 0x8048468: main (in /home/pjones/ghash)
==31032== 
==31032== 
==31032== 3,564 bytes in 4 blocks are still reachable in loss record 2 of 2
==31032==    at 0x401F705: calloc (vg_replace_malloc.c:279)
==31032==    by 0x40666ED: g_malloc0 (gmem.c:150)
==31032==    by 0x4075E7E: g_slice_init_nomessage (gslice.c:317)
==31032==    by 0x407638B: g_slice_alloc (gslice.c:347)
==31032==    by 0x4052B18: g_hash_table_new_full (ghash.c:139)
==31032==    by 0x4052B97: g_hash_table_new (ghash.c:110)
==31032==    by 0x8048468: main (in /home/pjones/ghash)
==31032== 
==31032== LEAK SUMMARY:
==31032==    definitely lost: 0 bytes in 0 blocks.
==31032==      possibly lost: 0 bytes in 0 blocks.
==31032==    still reachable: 5,580 bytes in 8 blocks.
==31032==         suppressed: 0 bytes in 0 blocks.