I honestly don't know what a safe dosage is for chins, though I know my roommate's medium-sized dog used to take 100mg three times a day.
Fyi, 0.35mL is a measure of volume rather than the actual dosage of the drug itself. In other words, it measures the liquid volume of the solution you give to him. The actual drug dosage would depend on how concentrated the solution is. The bottle should tell you what percentage of gabapentin is in the solution (usually something like "X% suspension"). Unless the solution is really concentrated, that doesn't sound like a huge dose to me, and gabapentin is generally considered a pretty "safe" drug in terms of the risk of overdose. But again, I don't really know anything about gaba and chins specifically.