I cannot use pgfgantt with the tikzposter document class, and I think it is due to a bug of one of the two packages. To be short, I have the following error:
! Package pgf Error: Sorry, the requested layer 'background' is not part of the layer list. Please verify that you provided \pgfsetlayers and that 'background ' is part of this list.
I have this error only when using the tikzposter document class, and I suspect that the problem comes from a variable used by the two packages, with the same name "background"... I have attached a MWE to demonstrate the problem: if you uncomment the pgfgantt code, I guess you should obtain the same error as me. I am compiling with the latest version of Tex (2018), that I reinstalled just now (I was using the version 2017 previously, and it was also not working).
A similar problem has actually been pointed out one year ago, and it has not been solved: https://tex.stackexchange.com/questions/372706/pgfgantt-gives-error-when-used-with-pgfplots A comment to this question suspects that the problem is due to the pgfplots version, but it is not the case for me since I am using the latest available. Also I do not have this problem with a regular article or beamer document class.
So I am not an expert of TeX but my understanding of the problem is that there is a conflict on the background variable. In any case if you know a fix or a workaround to solve this error, I am interested!