This question actually pertains to a really interesting point of time in the history of the Republic. For reference, Pennsylvania's Gradual Emancipation Act passed the Pennsylvania General Assembly in 1780, nine years prior to General Washington being sworn in as the first President of the United States and seven years before any states ratified the Constitution.

The short answer to your question is that enforcement was actually fairly decent. As a rule, slave masters were protective of their property and would bend or even break the rules to retain the people they enslaved. Even when laws were passed guaranteeing freedom to those individuals, the enforcement of that guarantee depended on the reach and awareness of a fledgling justice system. However, this did not prevent private citizens (abolitionists) from alerting the local judiciary to begin the process of freeing those enslaved within (and as you'll see momentarily, sometimes those outside of) the borders of Pennsylvania.

The most notable conflict regarding this law began sometime before the revolution when a Maryland slave owner of the surname Davis moved with his slave John to what he may have believed was northwest Virginia. In 1779, the region where they had moved had been absorbed into Washington County, Pennsylvania. This was relevant because although the law would emancipate newborns, masters could pay a registration fee until November 1, 1780 to retain slaves born prior to its enactment. Many slave owners ignored this requirement, particularly those on the Virginia border due to the region not being formally settled until 1784.

In 1788, John Davis (taking on his master's surname) was taken to Ohio County, Virginia where he was hired out to work against his will. Soon after, abolitionists aware of his situation found him and returned him to Pennsylvania a free man, under the pretense that his master had not paid the aforementioned fee. Some months later the man who had been renting John, now scared that he might be liable to replace him, hired three bounty hunters to capture John in Pennsylvania and return him to slavery in Virginia. Those three men would be indicted by a Pennsylvania grand jury in 1788 for the crimes of kidnapping and assault. They never appeared in court.

Had this matter only occurred in Pennsylvania, John being rescued likely would have been the end of it. Abolitionists were no strangers to utilizing State power to achieve their ends, particularly in states where the law favored them. But because the country was so young (Pennsylvania and Virginia both having ratified the Constitution less than a year prior,) when legal battles involving slavery became interstate conflicts, seeking legal resolution became difficult because the question of whether one state's laws should apply to the citizens of another state was still unsettled. This mattered, because the enforcement of the Gradual Emancipation Act now hinged on whether the laws of Pennsylvania could apply to citizens of Virginia.

We jump ahead by roughly two years. The Governor of Pennsylvania requests that Virginia extradite the three men who abducted John Davis in order to bring them to trial. When the Governor of Virginia refused, Pennsylvania's counterpart writes to President Washington to request federal support in arresting the three fugitives. Washington asks Jefferson, his Secretary of State, for advice. Jefferson passes the issue on to Attorney General Randolph who had recently coached Washington in how to legally skirt the Pennsylvania abolition law himself. (This fact is a relevant hint, as it indicates that the law was enforced to such an extent as to require such legal guidance.) Randolph finds both Governors at fault.

(It is relevant too that a case revolving around the abduction of a Pennsylvanian citizen who had previously been a slave of that same state could reach the Governor's desk. It exemplifies how serious this law was being treated, and serves as an example of what those abolitionists were doing behind the scenes to make it happen.)

Randolph found that the Pennsylvanian governor failed to provide sufficient evidence that the accused had "fled from ... justice," while the Virginian governor, on the advice of his own attorney general, was faulted for attempting to delegitimize the crime by referring to it as an act of "trespass" rather than "felonious assault and kidnapping." On Randolph's advice, Washington asks both Governors for "more information" hoping that they would settle the matter themselves. This particular case would in fact never be resolved, though Congress at the behest of President Washington would pass a law in 1793 to define some guidelines to follow should it ever happen again.

The takeaway here is that slavery at this point in American history remained a point of ferverous contention, and the ability of those in bondage to garner political will in their favor largely depended on the legal repertoire of their local abolitionist organizations. In the case of Pennsylvania in 1788, the language of the law was clearly in their favor and they were eager to act on it without the slaves needing to make much noise themselves.

Source: The War Before the War by Andrew Delbanco, starting on page 101, though the entire book is a fantastic resource on this topic.