"But when the fullness of time had come,"
Have you noticed that as time passes in your own life ....days, months, years become shorter? When you were a child, in school, days, weeks, months appeared to creep by ...until Summer break. As you grew older ...those same time-frames nearly blur in comparison. Clearly..... days or months or years have not shortened, or lengthened, or changed at all ...mathematically speaking. So what has changed? I submit that our own, relative, perception of time is what has changed.
"The years of our life are seventy, or even by reason of strength eighty; yet their span is but toil and trouble; they are soon gone, and we fly away."
(Psalm 90:10 ESV)
"For a thousand years in your sight are but as yesterday when it is past, or as a watch in the night."
(Psalm 90:4 ESV)
"God sent forth his Son, born of woman, born under the law, to redeem those who were under the law,"
God sent Christ to "buy back" those under the sin penalty of the law.
"for all have sinned and fall short of the glory of God, and are justified by his grace as a gift, through the redemption that is in Christ Jesus," (Rom 3:23, 24)
"so that we might receive adoption as sons"
"The adoption metaphor was used primarily in two contexts in Roman culture. In Roman law, adoption was very difficult. A long, involved and expensive legal procedure, once enacted adoption afforded several special rights and privileges: (1) all debts were cancelled; (2) all criminal charges were dropped; (3) they could not be legally put to death by their new father; and (4) they could not be disinherited by their new father. In legal terms, they were a completely new person. Paul was alluding to the believers’ security in Christ by using this Roman legal procedure (cf. Rom. 8:15, 23). When a father publicly adopted a son, he officially and permanently became his heir." (Bob Utley - You Can Understand The Bible!)
"And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!" So you are no longer a slave, but a son, and if a son, then an heir through God."
We have "inherited" the right, the privilege, to call Him "daddy", Father.
Christ' "gift" to you and I is that we are no longer stamped with the title, and baggage, of "slave" to sin. INSTEAD... we, as Roman culture practiced, as loving adoptive parents today practice, are heirs to the Kingdom which Christ knows.
"In my Father's house are many rooms. If it were not so, would I have told you that I go to prepare a place for you?
And if I go and prepare a place for you, I will come again and will take you to myself, that where I am you may be also."
(John 14:2, 3 ESV)