If you're planning to dimple the barrel, a dedicated jig is certainly the way to go. Using the gas block as a jig is a horrible idea. First of all, the dedicated jig lines itself up on the gas port to make sure the dimple is in the right spot. Secondly, putting a spinning bit into the screw hole of a gas block greatly increases the chances of ruining the threads on the gas block. And you're not saving any time. It's the same number of steps each way bit the jig saves you a few minutes. You lock the gas block or jig to the barrel. With the jig, the screw automatically centers itself on the gas port. With the gas block, you have to guess and then clamp the gas block down or take more time to ensure the gas block is in the exact right position. After you drill the dimple, you have to remove the jig or gas block to clean up the shavings. At that point, you might as well have used the jig.
That’s the main issue with not using a jig. They have no means of confirming alignment of the port and block, but also have no way to locate where the dimple should be. Using the gas block as a template is a bad idea, because again, there is no way to accurately locate the dimple location. I’ve seen multiple examples of dimpled barrels done improperly. A bore scope is inserted and can show the block is misaligned. Best way to do it is by locating the gas port and drilling a dimple opposite of it. That simply can’t be done accurately using this gas block as a template.