'Gassed' has two main slang meanings. The British/multicultural London English sense is 'excited' or 'hyped up' — 'I'm gassed for the concert' = 'I'm excited for the concert'. The American slang sense is being praised excessively, often in a way that inflates someone's ego ('they gassed him up too much'). If your teen says they're gassed, they are excited; if they say someone got gassed up, they got hyped/praised. Both are harmless conversational vocabulary.