I have received a gift from my son-in-law, who lives in the UK, of $200,000, do I pay taxes on that gift?

[macuser_22]

Gifts received are not taxable reguardless of the amount.

You would probably be required to file a 3520 form however, since it is over $100,000.

https://www.irs.gov/businesses/gifts-from-foreign-person

[Opus 17]

If your son-in-law is a "US person" (you aren't clear on this) for income tax purposes, he must report the gift.  No gift tax is owed unless his lifetime gifts and estate is more than $11 million.  

Gifts received by a US person are only taxable to the recipient under rare circumstances, such as if your son-in-law gifted you a piece of US investment or business property or certain other physical assets that are within the US.  Money is not a physical asset and so gifts of money are never taxable to the recipient.