Interesting, however we don't know very much about 10th Century Poland
Oh yes, we know enough. We know that Latin came to Poland via Bohemia, with St. Adalbert, his monks and baptism of Poland in 966. Written annals began to be generated in the late 10th century; first Polish Christian rulers were considered literate and educated. The secular annals later served as a basis for chronicles, such as Chronicle by Gallus Anonymus (around 1112-1116) or Polish Chronicles by Wincenty Kadłubek (ca 1202).
Here is a nice and short introduction to literary background during Middle Ages in Poland. The author, professor Mikos is also a renown translator from Latin and Polish and an author of several anthologies of Polish literature (in English), including "Medieval Literature of Poland".
staropolska.pl/ang/middleages/Mikos_middle/Literary_m.html
Below is a short excerpt from Chronicles of Gallus Anonymus, a part of that anthology. In this verse Poles are overjoyed after gaining access to Pomerania and Baltic Sea.
Agitabant patres nostri cervos, apros, capreas,
Hii venantur monstra maris et opes aequoreas.
Nasi ojce na jelenie urządzali polowanie,
A my skarby i potwory łowim, skryte w oceanie.
Our fathers hunted for deer,
But we hunt for treasures and monsters, hidden in the ocean.
and the way they spoke can only be guessed at.
How is it relevant? But I know enough about four schools of Latin pronunciation, including the funny English way. :-)
As far as we know, He spoke neither Latin nor Polish.
:-) But any monk worth his own black cassock had to know the Latin phrase "Gratis accepistis, gratis date" by heart. And from there, there is just one step to vernacular Polish. Same as with other Latin phrases, like "Deo Gracias". In Catholic Poland "Niech będzie pochwalony (Jezus Chrystus)" and "Na wieki wieków. Amen" were more popular for ages than the secular "Dzieńdobry", especially at the countrysite. "Szczęść Boże" and "Bóg zapłać" are also direct translations of Latin phrases.
I checked several Polish dictionaries, and one German source, and none ever mentions English parentage in the etymology chain of the word "gratis" either in Polish or in German. They all refer directly to the Latin original word "gratiis".
So sorry, your hypothesis here is quite weak. England simply was not in a center of Poland's cultural interests during medieval times. Italy was, France was, Germany had to be, as a close neighbour. But we just traded with you in Polish grain, timber and other produce.
I followed the Jan Brożek "Gratis" document, which I previously described, and I found it interesting enough to quote a passage from an article devoted to him. As you can see, people were flogged, "Gratis" was burnt at stake. So I rest my case here. :-)
In 1625, an unsigned pamphlet was printed and distributed in Kraków. It opened with the word Gratis. The pamphlet deals with the allegedly free (gratis) education offered by the Jesuits. The author argues that the claim of a free education is false. The author was, as it turned out, Brożek, and the pamphlet was his contribution in the struggle agains Jesuits. However, this episode ended badly. The city guards raided the printer's shop, the owner was arrested, flogged at the pillory and banned from Kraków. Gratis was burned at the pillory but Brożek was spared because the Academy defended him firmly.
gallusanonymus.fr/resources/BrozekPIASA_fin.pdf