What does "GR" mean on a pcb
Chances are they are 4-diode rectifier bridges. They might have the symbols ~ for AC input, and the symbols + and - for DC output.  

EDIT: Now that you have uploaded the picture, they're definitely 4-diode rectifier bridges. The datasheet for VM28 says they are rated for 1A curent and 200V peak reverse voltage. Most of us could have guessed it was a 1A rectifier, based on the chip size and the lack of any additional cooling provision.