Typically, where authors write a software article (as the one your DOI links to) they prefer the article to be cited; the G-power developers are explicit about this: https://www.psychologie.hhu.de/arbeitsgruppen/allgemeine-psychologie-und-arbeitspsychologie/gpower#c423889

You can choose to also add the software manually and cite it in addition to the article(s) - that's particularly important for software where version numbers matter, which wouldn't seem to be the case here: G-power has been on the same version for 5 years.

My typical approach with software versions is to note the version as a prefix on the in-text citation and then cite the software article in the bibliography.