Update for any future googlers:
I eventually used the gasspasser thing after my baby continued to seem to have trapped gas. She was struggling to sleep and was hugely overtired as had been like this for two days. I researched it first and found some evaluations. One of them, on Science Based Medicine, said that it probably wouldn't help but wouldn't do any harm if only used occasionally (only occasionally because babies need to build muscles and get used to passing wind themselves). I also found repeatedly good reviews, both reviews of purchases and a thread on Reddit. The only bad reviews I found said that it did nothing, so I thought I'd give it a go, especially as I researched the flatus tube things for adults and found it's a pretty tested way of relieving wind.
My husband and I got the nappy ready and used Vaseline to lubricate it a bit, like a rectal thermometer. We were really worried about hurting her, but it went in easily and she was not at all bothered by it. We slowly turned it round a bit and then all this wind just came out, so much wind (also some poo, like the packet said might happen). She instantly seemed relieved and is now sleeping in my husband's arms. So, all I can say is that it seemed to work, my baby did not seem at all bothered by it, and she is now sleeping after being unable to sleep for two days.
For any future googlers, apparently you are not supposed to use it that often, though, so that the baby's body learns to manage wind on its own. For us, we tried it because our baby was so bothered by wind these last two days, far more than normal. We don't be using it again unless we are desperate.
It seems to have been an unpopular idea, but all I can say now that I've done it is that I am glad I tried it. Obviously, each to their own, but wanted to post for anybody else trying to find real experiences of it like I was.