One of the first light instruments for the Giant Magellan Telescope (GMT) will be the GMT-Consortium Large Earth
Finder (G-CLEF). It is an optical band echelle spectrograph that is fiber fed to enable high stability. One of the key
capabilities of G-CLEF will be its extremely precise radial velocity (PRV) measurement capability. The RV precision
goal is 10 cm/sec, which is expected to be achieved with advanced calibration methods and the use of the GMT adaptive
optics system. G-CLEF, as part of the GMT suite of instruments, is being designed within GMT's automated
requirements management system. This includes requirements flow down, traceability, error budgeting, and systems
compliance. Error budgeting is being employed extensively to help manage G-CLEF technical requirements and ensure
that the top level requirements are met efficiently. In this paper we discuss the G-CLEF error budgeting process,
concentrating on the PRV precision and instrument throughput budgets. The PRV error budgeting process is covered in
detail, as we are taking a detailed systems error budgeting approach to the PRV requirement. This has proven
particularly challenging, as the precise measurement of radial velocity is a complex process, with error sources that are
difficult to model and a complex calibration process that is integral to the RV measurement. The PRV budget combines
traditional modeling and analysis techniques, where applicable, with semi-empirical techniques, as necessary.
Extrapolation from existing PRV instruments is also used in the budgeting process.