Gr. is short for 'Group Number'. Railway electrification works are carried out in different formats. For example, a single contractor might handle all works (OHE, Sub-station, Signal and Telecom etc) for a particular section, or separate contracts may be awarded for OHE works and other works. There are usually 4 different methods followed in electrification in terms of type and scope of contracts.
For this sake, each electrification project is split into groups, with each group having its own method of awarding contracts and each group being handled by a different set of contractors or even different agencies (CORE and RVNL).