Deductions & credits

The simple answer to your basic question is NO. You do NOT pay gift taxes on the houses that become your sole property. You do not pay federal inheritance tax, either, unless you mother's estate is worth more than $5.49 million. Furthermore, gift tax is paid by the giver, not the recipient**.

"For 2017, the estate and gift tax exemption is $5.49 million per individual, up from $5.45 million in 2016. That means an individual can leave $5.49 million to heirs and pay no federal estate or gift tax." 

There are other issues, inherent in your question. Hence the other information provided by we commentators.

Half your cost basis "steps up" on your mother's death. It's best explained by example. Your mom bought a house in 1990 for $100,000. In 2000, she deeded half to you. Your cost basis in your half is still $50,000 (the gift recipient's cost basis is the giver's basis) and her basis in her half is $50K. When she dies, the house is worth $200,000. Your cost basis is now $150,000 (your original $50K + the $100K stepped up value of her half). 

**"Gift Tax" is somewhat of a misnomer.  Even though a gift tax return may be required, very few people ever actually pay federal gift tax. The purpose of the gift tax return is usually only to document a reduction in the allowable estate tax exemption.Your mother probably should have filed a gift tax return when she deeded you half in 2000.