Gad! Egad! We use it any time we dashing well please! Haha, no it's not popular. I'd expect to hear this from an old-world toff.

P.G. Wodehouse has a few "By Gad"'s and "By Jove"'s scattered around in his stories. The interjections are old-fashioned but certainly not ancient. Of course, for people who haven't read Wodehouse or O'Henry these may seem prehistoric or even completely alien. Wodehouse is quite 20th century, by the way, and so are his story settings.

How do you, British, use the interjection "gad"? We don't use it at all. Gad! means astonishment, right? Wrong Is this word popular in spoken English? No, it doesn't even exist as a word. In the 16th -18th centuries, 'egad' and similar were exclamations. 'Gad' was informal pronunciation of 'God' in Shakespeare's time, but nobody has said this for several hundred years. I presume you've been studying Shakespeare! If not, what are the interjections with the similar meaning? The modern-day equivalent, especially for teenage girls, must be the all-purpose 'Oh my God'. Interjections are very personal things and vary according to region, age and gender - so I don't feel qualified to say what would be the most appropriate way for you to express surprise. I'll leave it to an English-speaking teenage boy to suggest something, though I suspect it might be a 'four-letter word'. Most teenage boys I come across these days show surprise with a short word starting with 'sh' or 'f', or variations on them.

Used a lot in 18th century literature Gad-zooks as a swear word (probably not spelt correctly) then in the early 20th century one would be a gad-about-town which means someone wealthy who goes to a lot of parties. Gad now is not used very much. There is a gad-fly a busy person but again not used much except by an older generation. Surprise; probably the most likely is "what" followed by "what the" or "what the @@@" or calling for one's god. :-)

'Gad' is not an interjection I have ever heard. It sounds extremely strange and I think it is safe to say it is not used. It sounds like it is an archaic form of 'God' from back when blaspheming was considered a sin. Nowadays, as most people are not religious, no one really cares about that.