I understand that Gerundive is a verbal adjective and will (usually?) have an associated noun that it describes. The Plebs believe that the king of Rome to succeed Romulus should come from amongst them. The patricians are inclined to make concessions.
Cum hoc sensissent patres, populo [plebs] concedendum esse censuerunt - ita tamen ut non plus iuris darent quam retinerent. Decreverunt enim ut, cum populus regem creavisset, patres auctores fierent.
is 'se' the implied word described by the gerundive 'concedendum'? When the elders understood this they thought that they should be conciliatory - accordingly, however, they would not make any new laws than they already had made [retinerent?]. They allowed that the Plebians could nominate a king, the Patricians would be those who would crown him. [ this is very idiomatic and I may be completely wrong...Orberg helpfully hints at this point: [auctor fieri : regem creatum probare]