It seems it won't work I tried searching in google or even here I can't find any post that 6gb works in grin miner.

Most of the miner is above 7gb that works to mine grin based on what I heard from others.

However, there are some newbie posting that there is a mining OS that you can mine grin with 6gb GPUs or below. Not tested yet but I don't trust newbie about the OS he offer.