Endocet (brand name Percocet) is a combination of acetaminophen (like Tylenol) and oxycodone (an opioid given for moderate to severe pain).  Gabapentin (brand name Neurontin) is an anticonvulsant that has been found to be effective for some types of pain, including neuropathic pain.  Tramadol (brand name Ultram) is an opioid-like pain reliever used to treat moderate to severe pain. 

A couple of differences: 

•  Patients often build up a tolerence to opioid medications like endocet, requiring increasing dosages to control the pain while they do not seem to build a tolerence to gabapentin and tramadol.

•  Opioid medications are often not as effective for some types of chronic pain (like fibromyalgia) as drugs like gabapentin and tramadol.

You can read more about each of these medications here:

Endocet (Percocet)

Gabapentin (Neurontin)

Tramadol (Ultram)