The gev- stem is a class 3 adjective. https://wiki.dothraki.org/High_Valyrian_Adjectives#Prepositive_3
Gevȳr would fit with the terrestrial genitive for that class of adjective.
As the noun is in the locative case, I believe it should read Gevȳro dāriot glaesāt! according to the table I have linked to.