Don't take gabapentin with tramadol. Tramadol is an opioid, it has no cross tolerance with weed. Tramadol, like previously stated, is hard to dose. It comes on slow and sequential doses potentiate each other. I would say start with just 50mg, see how your body handles it, if that works out wait at least a day and then try taking 100mg's spread out over a couple hours.
Some people don't have adequate liver enzymes to efficiently convert tramadol to its more potent prodrug O-desmethyltramadol, so you may find that no amount of tramadol will satisfy you.
Obligatory warning, tramadol lowers the seizure threshold. This means you are more prone to seizures on tramadol, if you have ever had a seizure be extremely cautious with tram. Under no circumstance consume more than 300mg, especially with no tolerance.