x86_64 assembly, gtk3, 21 lines / 19 instructions

build with e.g. gcc hw.s `pkg-config --libs gtk+-3.0`

  .globl main;main:
  push %rbx
  xor %esi, %esi
  xor %edi, %edi
  call gtk_init@PLT
  lea .HW(%rip), %r8
  xor %edx, %edx
  xor %esi, %esi
  mov $2, %ecx
  xor %edi, %edi
  xor %eax, %eax
  call gtk_message_dialog_new@PLT
  mov %rax, %rbx
  mov %rax, %rdi
  call gtk_dialog_run@PLT
  mov %rbx, %rdi
  call gtk_widget_destroy@PLT
  xor %eax, %eax
  pop %rbx
  ret
  .HW:.string "Hello World!"