I created a grey rectangle in Sketch v45 and put the word "TEXT" on it. Selected a red for the text colour and saved my file.
Imported my Sketch file into the Mac version of Gravit Deisgner 3.1.2 and there's no colour on my text. I don't think a test can get much more basic than that.
I've given up trying to get any of my other Sketch files into Gravit Designer, none of them load, they all show "Load failed..." Which, if I could find a way to post screenshots to this forum software, I'd show people.
All my Sketch files are V42 but even if I create an empty Sketch v45 document and copy / paste my artboard(s) into the empty v45 document & save it - the files never import into Gravit Designer. I don't have any symbols and the designs are far from complex.
Such a shame.