Do we need to install a Grounding Electrode System for Grid Tied transformerless inverters?
Your question implies that there is a utility service, which should already have a GES. The code has allowed the premises GES to provide both AC and DC grounding since at least the 2008 cycle. You should not have to install a new GES or do any work on any GES
unless the AHJ requires you to bring an older premises GES up to current code (e.g. install a rod where there was only a water bond).
Regarding transformerless inverters, the 2014 code makes a distinction between grounded and ungrounded DC systems. A transformless inverter indicates an ungrounded system. (An transformer inverter usually, but not always, indicates a grounded system. A grounded system always has a transformer in the inverter, unless it's bipolar.)
For an ungrounded system, the 2014 code allows equipment grounding to serve as the reference for the inverter GFDI and states that the grounding conductor from the inverter to the 'associated AC equipment' can be sized to 250.122. This could be read to imply that a transformerless inverter only requires an equipment grounding conductor, or that the conductor has to be installed like a GEC (bonding bushings, continous, etc..) but can be sized smaller than a minimum 8 awg.
Previous codes make no such distinctions.
There's also 690.47(D), which has been in and out of the code, but which is for the array and not 'for the inverter.' If your AHJ requires you to follow that (see other thread started today), then it will probably make no difference what type of inverter you have.