If a real estate firm accepts BTC without conversion to USD first, does one owe capital gains on taxes?
This will depend on the country you are in but in most countries, it will work like this:
Receiving payment in BTC will be taxed in the same way as if you were receiving fiat. So if you receive 1 BTC as payment and the price of BTC is $40,000 at the time, then you would be taxed in the same way as if you had just received $40,000 in USD.
NOTE: I am not a tax or financial advisor and this is not tax advice