General recommendation:
Take the train from Darmstadt main railway station to Wixhausen (2nd stop) and there change to S3 in direction to Bad Soden
https://www.gsi.de/en/directions/train
Schedule S3, in direction to Bad Soden (page 12 following)
https://www.rmv.de/c/fileadmin/import/timetable/RMV_Linienfahrplan_S3-S4_ab_12.12.21_bis_10.12.22.pdf
Then, take the G bus to GSI, please see...
https://www.rmv.de/c/fileadmin/documents/Stationsplaene/Darmstadt-Wixhausen.pdf
Schedule G-Bus:
https://www.heagmobilo.de/sites/default/files/media/Li.%20G_SEV%20Luisenplatz-Nordbad_g%C3%BCltig%20ab%2025.04.2022.pdf
Journey planner public transportation:
https://www.rmv.de/c/en/homepage
