Will a .ino Arduino Sketch compile directly on GCC-AVR?
See my answer here: Classes and objects: how many and which file types do I actually need to use them? - specifically: How the IDE organizes things.


  I know that at a minimum you have to include the Arduino header file


Yes you would need to do that.


  but beyond that, what would cause a compilation error if compiling said .ino to .cpp file using, say, for instance GCC-AVR.


The IDE generates function prototypes for you. Code in a .ino file may or may not need this (it probably will unless the author is disciplined enough to code in the usual C++ way and do them themselves).



If the "sketch" contains other files (eg. other .ino, .c or .cpp files) then these would need to be incorporated in the compile process as I describe in my answer mentioned above.

Also you would need to (compile and) link in any libraries used by the sketch.



You haven't asked about the linking side of things, but naturally the various files, as compiled, need to be linked together, and then turned into a .elf and .hex file for uploading purposes. See below.



Example makefile

Based on the IDE output I made a simple makefile a while back:

#
# Simple Arduino Makefile
#
# Author: Nick Gammon
# Date: 18th March 2015

# where you installed the Arduino app
ARDUINO_DIR = C:/Documents and Settings/Nick/Desktop/arduino-1.0.6/

# various programs
CC = "$(ARDUINO_DIR)hardware/tools/avr/bin/avr-gcc"
CPP = "$(ARDUINO_DIR)hardware/tools/avr/bin/avr-g++"
AR = "$(ARDUINO_DIR)hardware/tools/avr/bin/avr-ar"
OBJ_COPY = "$(ARDUINO_DIR)hardware/tools/avr/bin/avr-objcopy"

MAIN_SKETCH = Blink.cpp

# compile flags for g++ and gcc

# may need to change these
F_CPU = 16000000
MCU = atmega328p

# compile flags
GENERAL_FLAGS = -c -g -Os -Wall -ffunction-sections -fdata-sections -mmcu=$(MCU) -DF_CPU=$(F_CPU)L -MMD -DUSB_VID=null -DUSB_PID=null -DARDUINO=106
CPP_FLAGS = $(GENERAL_FLAGS) -fno-exceptions
CC_FLAGS  = $(GENERAL_FLAGS)

# location of include files
INCLUDE_FILES = "-I$(ARDUINO_DIR)hardware/arduino/cores/arduino" "-I$(ARDUINO_DIR)hardware/arduino/variants/standard"

# library sources
LIBRARY_DIR = "$(ARDUINO_DIR)hardware/arduino/cores/arduino/"

build:

    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(MAIN_SKETCH) -o $(MAIN_SKETCH).o
    $(CC) $(CC_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)avr-libc/malloc.c -o malloc.c.o 
    $(CC) $(CC_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)avr-libc/realloc.c -o realloc.c.o 
    $(CC) $(CC_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)WInterrupts.c -o WInterrupts.c.o 
    $(CC) $(CC_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)wiring.c -o wiring.c.o 
    $(CC) $(CC_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)wiring_analog.c -o wiring_analog.c.o 
    $(CC) $(CC_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)wiring_digital.c -o wiring_digital.c.o 
    $(CC) $(CC_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)wiring_pulse.c -o wiring_pulse.c.o 
    $(CC) $(CC_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)wiring_shift.c -o wiring_shift.c.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)CDC.cpp -o CDC.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)HardwareSerial.cpp -o HardwareSerial.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)HID.cpp -o HID.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)IPAddress.cpp -o IPAddress.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)main.cpp -o main.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)new.cpp -o new.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)Print.cpp -o Print.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)Stream.cpp -o Stream.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)Tone.cpp -o Tone.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)USBCore.cpp -o USBCore.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)WMath.cpp -o WMath.cpp.o 
    $(CPP) $(CPP_FLAGS) $(INCLUDE_FILES) $(LIBRARY_DIR)WString.cpp -o WString.cpp.o 
    rm core.a
    $(AR) rcs core.a malloc.c.o 
    $(AR) rcs core.a realloc.c.o 
    $(AR) rcs core.a WInterrupts.c.o 
    $(AR) rcs core.a wiring.c.o 
    $(AR) rcs core.a wiring_analog.c.o 
    $(AR) rcs core.a wiring_digital.c.o 
    $(AR) rcs core.a wiring_pulse.c.o 
    $(AR) rcs core.a wiring_shift.c.o 
    $(AR) rcs core.a CDC.cpp.o 
    $(AR) rcs core.a HardwareSerial.cpp.o 
    $(AR) rcs core.a HID.cpp.o 
    $(AR) rcs core.a IPAddress.cpp.o 
    $(AR) rcs core.a main.cpp.o 
    $(AR) rcs core.a new.cpp.o 
    $(AR) rcs core.a Print.cpp.o 
    $(AR) rcs core.a Stream.cpp.o 
    $(AR) rcs core.a Tone.cpp.o 
    $(AR) rcs core.a USBCore.cpp.o 
    $(AR) rcs core.a WMath.cpp.o 
    $(AR) rcs core.a WString.cpp.o 
    $(CC) -Os -Wl,--gc-sections -mmcu=$(MCU) -o $(MAIN_SKETCH).elf $(MAIN_SKETCH).o core.a -lm 
    $(OBJ_COPY) -O ihex -j .eeprom --set-section-flags=.eeprom=alloc,load --no-change-warnings --change-section-lma .eeprom=0 $(MAIN_SKETCH).elf $(MAIN_SKETCH).eep 
    $(OBJ_COPY) -O ihex -R .eeprom $(MAIN_SKETCH).elf $(MAIN_SKETCH).hex 


In that particular case, the .ino file compiled without any problems after renaming it to Blink.cpp and adding this line:

#include &lt;Arduino.h&gt;