"A Gentile (stranger) is by definition not "one that is born in the land", unless he is a slave in Israel, that was bought with silver and circumcised by their Israelite master. Once circumcised, that slave may eat of the Passover sacrifice. As Gentiles grafted into Israel, we were not bought by silver or gold. Peter explains:"
In the Torah a stranger (ger) is someone who lives in the borders of Israel but is not a citizen, is not an Israelite. It would be like someone coming to be a resident in America from another country and working there but not permanently, unless later they changed their mind and applied for it and were accepted and went through the process of becoming a citizen, which in the Torah would be getting circumcised and observing the Passover.
There is nothing here about slaves. And there is nothing here about a purchase. That matter of slaves wanting to keep the Passover was already finished. It is now talking about strangers, which are not limited to slaves.
It goes on to state that NO uncircumcised person is allowed to eat of the Passover.
Abraham and his whole household getting circumcised is further proof that circumcision in not limited to 8 day year old children. That is just when it is mandatory for children to be circumcised and not any earlier because any day before that and they will bleed to that. And circumcision is important as the sign of the covenant and that is why one is not to delay it extra days after or else face being cut off.
It is true that God specifically states in the Torah that no uncircumcised person is allowed to EAT the Passover sacrifice. More specifically:
Then He continues to explain the ordinance of the slave foreigner (who was bought with silver) partaking in Passover. Then God says:
I read this as a continuation of the exception of the law "no stranger shall eat thereof", that slave foreigners who are sojourning with the Israelites must become circumcised before eating the Passover sacrifice.
God gave Abraham the covenant of circumcision:
The covenant between God and Abraham and his decedents is eighth day circumcision.
In addition to the covenant, God also instructed Abraham to circumcise his adult males and his foreign slaves bought with silver. This is consistent with the passage in Exodus 12. It is not a command for Gentile believers to become circumcised as adults.