Following ATR-19193, some code changes in the configuration of FTK b-jet chains are needed in order to support GSC. This has been tested in 21.3 by adding some FTK+GSC b-jet chains in the Physics v7 menu and with the following command:
trigtest.pl --cleardir --test AthenaTrigRDO_MC_pp_v7_no_prescale_toxAOD_FTK --rundir AthenaTrigRDO_MC_pp_v7_no_prescale_toxAOD_FTK --conf TriggerTest.conf
Additional code changes have to be applied for addressing ATR-19262, but I'll open another MR for that since it is a different issue.