The gas block is either held on with pins or with set screws. If yours is held in with pins, you need to punch them from left to right out of the sight completely. If set screws, I would just back them out completely.
Set screws may be held in with locktite, but pins are typically not. A hammer and punch to remove them completely will be in order. Some pins are tapered, and some strait. You need to be sure when you put them back in, they go back in the same way with taper pins.