It's a dialect/accent thing. The medial mh/bh in some dialects/accents sounds like 'h' and in others like 'v'. So it will sound either like 'ga-hal' (or even 'gaal') or 'ga-val.' This distinction between aspiration (e.g. bh as 'h') and lenition (e.g. bh as 'v') is a very common marker of dialect in the Celtic languages.
Already answered above. For I had you would use the past tense 'Ghabh mi' for I was having you can use the past tense of the is verb. Basically the confusion is the translation of gabh. It's not a direct translation of 'have' (I'm not entirely sure that Gaelic has a verb for that, it tends to say things are at people or with them rather than they have them). The best translation in this context is 'to take'. So I was 'taking' another drink. The past tense form would be close to 'I took another drink'. It's one of the more flexible verbs so it won't mean that in all contexts but for finished actions in the past the past tense (which I don't think we've got to at this point) is better.