I have the Libre3 and have found the GMI values to be roughly inline with my A1C results. And my doctor claims that it's usually the case. As someone else suggested, the A1C is a three-month "average". So, to get a similar value, you must look at the GMI computed across three months of data. The 7-day, 14-day, or 30-day values will not be good estimates of your A1C.
And contrary to what other posters have read, I find the Libre3 very helpful with my type 2. It has been very helpful it showing me which foods are problematic/helpful as well as how exercise helps. Getting the immediate feedback has been great in helping me drive down my A1C far faster than would have been possible with an occasional finger prick.