Paul’s readers would have seen slave-guardians in their cities.  They were slaves, owned by their masters, but were trusted to have the responsibility of preparing the children of the house for adulthood.  They were not family members but were given authority over the children, even to the point of disciplining them.  However, when they reached maturity, the young people no longer needed their guardians, and they took their full role as adult members of the family.
 
Paul used this metaphor to illustrate the difference between the law which God gave to Moses, and eternal life which God gives to all who put their faith in Jesus Christ.  The law is like the slave-guardian; it was given temporary authority over God’s people until Jesus Christ died and rose again.  During that time it disciplined them and they were controlled by it.  But when Jesus came and people put their faith in Him, God accepted them as mature members of His family.  

But the language Paul uses is not so benign.  “… held in custody” and “… locked up” seem to imply that the slave-guardian’s behaviour is like that of a jailor.  The two functions of law are to define what is right and wrong, and to punish offenders (Romans 3:20).  There is no reward in law for right behaviour.  Spiritually, the law declares everybody to be in the wrong at some point because nobody except Jesus is always right.  But devout people, who rely on always doing what is right, lock themselves into perpetual failure.  Jesus said that people who sin are slaves to sin (John 8:34).  Sinners are always trapped in their sin, unless their sin is taken away by Jesus (Acts 13:38-39).

That was why Paul urged the churches in Galatia not to get bound up in religious rules again, but learn to trust the Lord Jesus who was crucified for their sins and raised for their justification (Romans 4:25).  Although the Old Testament law restrained bad behaviour to some extent, it could never bring anyone into God’s family.  But when Jesus died, He chose to be a sacrifice on our behalf, to satisfy Father God’s anger at mankind’s rebellion.  So, everybody who admits that they are sinners and believes that Jesus died and rose again for them, are made disciples of Jesus and given the right to be mature children of God (John 1:12).  If you believe this – rejoice and give thanks for your freedom in Christ … and tell somebody else the good news.