Translation from praxisvita.de: 

"Common side effects are: Viral infection Drowsiness, dizziness, incoordination Fatigue, fever Pneumonia, respiratory tract infections, urinary tract infections, other infections, ear infections Decrease in white blood cells Loss of appetite or increased appetite Aggressive behavior towards others, confusion, mood swings, depression, anxiety, nervousness, thought disorders Cramps, movement disorders, speech disorders, memory loss, tremors, sleep disorders, headaches, sensitive skin, decreased sensation, impaired coordination, unusual eye movements, increased, weakened or absent reflexes Blurred vision, double vision High blood pressure, flushing, or widening of the blood vessels Difficulty breathing, bronchitis, sore throat, cough, dry nose Vomiting, nausea, dental problems, inflammation of the gums, diarrhea, stomach pain, indigestion, constipation, dry mouth or throat, gas Joint pain, muscle pain, back pain, muscle twitching Erectile dysfunction Decrease in white blood cells, weight gain In addition, aggressive behavior and excessive physical activity were often reported in children in clinical studies."

And this are only the side effects which occur OFTEN !!!

"Rarely occurring side effects while taking gabapentin include: Heart pounding Swelling of the face, trunk and limbs Abnormal blood tests that indicate liver problems The following side effects have been reported since launch: Hallucinations (hallucinations) Movement disorders such as jerking, jerking movements and stiffness Ringing in the ears Inflammation of the pancreas Inflammation of the liver, yellowing of the skin and eyes Severe skin reactions that need immediate medical attention, swelling of the lips and face, rash and reddening of the skin, hair loss Acute kidney failure, urinary incontinence The medicine can cause a serious or life-threatening allergic reaction that may affect your skin or other parts of the body such as the liver or blood cells. You may or may not get a rash with this type of reaction. The allergic reaction may lead to your hospitalization or discontinuation of gabapentin treatment." So I think, one has to be VERY careful with this drug!

"I know it sounds daunting, but that list of side effects is to cover the manufacturer’s butt just in case." 

No. There is a scheme of what is meant by "often". And "often" means: At least one case of 10 --> 10 % and more. 

I already have a bad conscience because of the amputation. I don't want to completely ruin my dog.

Today I had to leave her alone for an hour and she was so excited that she peed on the carpet. Never did this before. Although she always had problems staying alone and was never really relaxed without me.

Apart from that, she was fine today. That's why I asked my neighbors to help me carry her down twice. But the second time may have been too much. Now she is lying around exhausted again.

No, I don't know when the stitches come out. Day 15 post op?

She seems to show less pain every day. Today is the last day with antibiotics. She walked very fast to the corner of our street where I put the poop in the garbage can. Walking up the stairs works fine with some help (not so fine for my back and my left knee). But still breathes very fast even after very small walks (from her bed to the kitchen).

Downstairs she still must be carried by my neighbours. Now I am looking for more videos with downstairs-walking threelegged dogs.