Help please? What is gabapentin? Does it cause weight gain and sleepiness?

It can. While it does not normally cause weight gain, it can cause water retention and fatigue/sleepiness leading to decreased activity. It is an antiseizure medication, a gaba analog, often used to treat neuropathic pain by means of increasing gaba, an inhibitory neurotransmitter, in the synaptic cleft (decreases peripheral neural activity).