I had a grip for my Nikon D300 and used it a lot, mainly for portrait shooting. It too was a knock-off but it was a particularly good one and it stopped working after a few months. 

Incidentally, when I looked at the GH5 in a local store here, the guy in the store said that it did have the 29-min limit and that it was to do with import taxes. Anything over that would be classed as a broadcast class camera and then a higher import duty kicks in. I'm not sure he was 100% on that, but it's moot anyway cos I got the Sony.