Looking at website of Guyanese Embassy in Brasilia, Brazil I'm not sure if they issue visas to foreigners:
We advise that Brazilians do not need a tourism visa to get into Guyana (...) Fees: touristic visa - R$45,00 (...)
Since they mention that Brazilians don't need a touristic visa and yet publish its price, I understand they issue visas to foreigners. There's also a visa form in English available on their website.
On the other hand, according to Polish MFA (information available in Polish only)
You can apply for a visa in Guyanese Embassy in Brussels or Caracas, Venezuela
My question is, as a citizen of Poland and resident of Brazil, can I get Guyanese visa in Brasilia, Brazil?