The DHIS2 documentation mentions that it supports mysql (https://docs.dhis2.org/2.28/en/implementer/html/installation.html), however thats the last point mysql is ever mentioned.
Does the current version really support mysql? If it does, will GIS still work?
First of all it is recommended to use postgres.
Secondly most of the testing and QA is done on instances with postgres.
Thirdly POST GIS extension is available only in postgres , which can cause a hurdle for you at later stage.
Fourthly , the GIS data points and boundaries are stored in a format which is better handled in postgres db structure.
Therefore please go with postgres and chill