Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Miss T ingested two 500mg Metformin pills, and one 300mg Gabapentin pill.
Metformin is used to keep blood sugar low in diabetics. It is sometimes used in cats, but the dose is much lower, 50mgs per cat. So she ingested roughly 20 times a normal dose. This is very concerning and could lead to very low blood sugar levels and seizures, as well as liver toxicity.
Gabapentin is sometimes used for pain relief in cats, especially those with neuropathy or nerve root pain. A usual dose is 1.3mgs per pound of body weight, so this is significantly higher than usual. It is unlikely to be harmful long term, but you can see vomiting, diarrhea, and sedation/incoordination from a high dose.
My main concern is the Metformin, that could be very toxic for her. Ideally she would be seen at an emergency clinic immediately to induce vomiting and be given activated charcoal to decrease absorption of any remaining drug. They may want to hospitalize her to keep an eye on her blood sugar and give supportive fluids. This is not something that is amenable to home treatment.
Please let me know if you have any further questions.