Rainforest Restaurant And Bar — billing

Address: Bandra East., Mumbai City, Maharashtra, 400051

Today me and my family went for a lunch in rainforest restaurant and bar in kurla phoenix mall.
It was a nice experience overall. Was served once at the start by the waiter when the food came. Then we only took our food the second time. Had to ask for water when the glass was empty. The thing that bothered me was the bill. Cgst and sgst 2.5% on food was charged which we have to pay.
Then they charged service charge 5% and 2.5% cgst and 2.5% sgst on the service charge again.
I asked the waiter abt the extra charges i. E. 2.5% cgst and sgst each on service charge. He told me thats our policy which we have put a board outside at the entrance. N he deducted only 10 rupees from it and we were charged total 10% of service charge on our bill. Our bill was abt 4329. My complaint is why are they charging so much of service charge from any customer? Is this even legal? Plus a separate cgst and sgst in the name of government in their own pockets.
I've attached images of todays bill. Also check the board they have displayed which mentions nothing abt the extra service charges.