Nevermind I managed to solve this in the end, for anybody else looking, solution is:
{=IF(ISERROR(INDEX('Gantt Chart'!$A$5:$AB$31,(SMALL(IF(INDIRECT("'Gantt Chart'!R5C"&$G$2+3,0):INDIRECT("'Gantt Chart'!R31C"&$G$2+3,0)>0,ROW('Gantt Chart'!$A$5:$A$31)),ROW(1:1)))-4,1)),"",INDEX('Gantt...