Good question. There is no tax due from the one giving even at that time. One could give away even say $1M at one time and not be due any tax payment at that time.

An inheritance tax will only be due at time of death if one gives away more than their lifetime exemption level (currently $15M per individual) and/or a combination of giving plus net worth of greater than $15M at death.

Say for example, one has given away $10M during his lifetime (no taxes at all ever paid by either party on this when it occurred) and has a net worth of $3M at the time of death. They owe no federal tax at all. Now, say that same individual had given $10M away during his lifetime (no taxes at all ever paid by either party at all on this when it occurred) and has a net worth of $10M at death. They/estate would owe federal inheritance tax at death on $5M if they died in 2026. ($10M gifting plus $10M net worth minus the $15M lifetime exemption) 

This is the basics of it.