"係噉" is simple, it just means "是這樣" (it is like this/ just like that) in Mandarin
The final particle "㗎嘞" is the tricky part. Cantonese use final particle much more extensively than Mandarin, many of them have no Mandarin equivalence
The final particle "㗎嘞" carry the following expressions:
- authoritative (the statement "係噉" is made with authority, don't argue about it)
Examples: [係噉㗎嘞], 老闆鐘意炒你就炒你 = [just like that], the boss would fire you if he felt like it
- routine (the statement "係噉" is true as always)
Examples: 佢個個禮拜日都要去打高爾夫, [係噉㗎嘞] = He have to go play golf every Sunday, [it is just like that]
- finality (the statement "係噉" is final, nothing you can do about it)
Examples: 今日午餐毎人一隻煎荷包蛋，半條臘腸, [係噉㗎嘞] = Everyone get a pan fired egg and half a sausage for today's lunch, [that's all/ it is just like that]
If you must translate "係噉㗎嘞" into Mandarin, I would suggest "就是这样的啦", but in reality, "就是这样的啦" cannot fully express all the meanings in "係噉㗎嘞".
Take example one "係噉㗎嘞, 老闆鐘意炒你就炒你" for example, instead of ""老闆愛解雇你就解雇你, 就是这样的啦", an interpretation of "老闆愛解雇你就解雇你, 你又能怎樣?/ 就是這簡單" would be closer to the original Cantonese sentence.
My conclusion is, the closest Mandarin equivalent of "係噉㗎嘞" is "就是这样的啦/了" but it is not 100% the same, because using final particle "㗎嘞" to express subtle messages I mentioned doesn't exist in Mandarin grammar