what would be the effects of huffing gas every weekend

Any time "gas" is insufflated, or "huffed"there is risk for aspiration, and subsequent pneumonitis, an acute inflammation of the lungs. However, it sounds like this is a question about regular use, albeit weekends. Prolonged abuse of hydrocarbons can result in white matter degeneration (leukoencephalopathy). Symptoms include memory impairment, personality changes, loss of coordination, speech impairment, learning disabilities, and hallucinations. Tremors and uncontrollable shaking can result after a long time of abuse. Effects can also focus on psychiatric aspects such as  impaired judgment, confusion, fright, hyperactivity, anxiety, acute psychosis, increased violence and aggressive behavior, depression, organic brain syndrome (ie, coarse tremor, staggering gait, speech problems, thought disorder), abuse, tolerance and dependence, hallucinations, decreased intelligence quotient, intoxication, mood disorder, dementia, and withdrawal. Finally, benzene, an ingredient in gasoline has been shown to lead to leukemia.