If the OP measures 30 V between the neutral and the ground wire in the box (not the ground in the receptacle which is not connected to anything), then it is clear that the ground wire is not bonded to the neutral in the panel, but could it be that the ground wire goes to a water pipe or to a ground rod?

To test whether the ground wire goes to ground in the panel put the digital meter in resistance mode and measure the resistance between the loose ground and the neutral (Not the hot!!! If you mistakenly connect to a hot in resistance mode, you will at the least blow a fuse in the meter and might damage the meter.) A low resistance between the ground wire (~0.1 - 0.3 ohm) means the ground wire goes to the panel and is bonded to the neutral there. That is what you hope for.

If the resistance is higher (say 1 ohm to 500 ohm), then that would mean the wire goes to a ground rod or to a water pipe, but is not bonded to the neutral in the panel. I think this wire would still be usable as a ground for a GFCI receptacle, but expert opinion will be needed.

If the resistance is high kilo- or mega-ohms, that would mean the ground wire is not connected to anything and is useless as a ground.

If the OP measures 30 V between the neutral and the ground wire in the box (not the ground in the receptacle which is not connected to anything), then it is clear that the ground wire is not bonded to the neutral in the panel, but could it be that the ground wire goes to a water pipe or to a ground rod?

To test whether the ground wire goes to ground in the panel and is bonded to the neutral put the digital meter in resistance mode and measure the resistance between the loose ground and the neutral (Not the hot!!! If you mistakenly connect to a hot in resistance mode, you will at the least blow a fuse in the meter and might damage the meter.) A low resistance between the ground wire (~0.1 - 0.3 ohm) means the ground wire goes to the panel and is bonded to the neutral there. That is what you hope for.

If the resistance is higher (say 1 ohm to 500 ohm), then that would mean the wire goes to a ground rod or to a water pipe, but is not bonded to the neutral in the panel. I think this wire would still be usable as a ground for a GFCI receptacle, but expert opinion will be needed.

If the resistance is high kilo- or mega-ohms, that would mean the ground wire is not connected to anything and is useless as a ground.

To test whether the ground wire goes to ground in the panel put the digital meter in resistance mode and measure the resistance between the loose ground and the neutral (Not the hot!!! If you mistakenly connect to a hot in resistance mode, you will at the least blow a fuse in the meter and might damage the meter.) A low resistance between the ground wire (~0.1 - 0.3 ohm) means the ground wire goes to the panel and is bonded to the neutral there. That is what you hope for.

If the resistance is higher (say 1 ohm to 500 ohm), then that would mean the wire goes to a ground rod or to a water pipe, but is not bonded to the neutral in the panel. I think this wire would still be usable as a ground for a GFCI receptacle, but expert opinion will be needed.

If the resistance is high kilo- or mega-ohms, that would mean the ground wire is not connected to anything and is useless as a ground.

If the OP measures 30 V between the neutral and the ground wire in the box (not the ground in the receptacle which is not connected to anything), then it is clear that the ground wire is not bonded to the neutral in the panel, but could it be that the ground wire goes to a water pipe or to a ground rod?

To test whether the ground wire goes to ground in the panel put the digital meter in resistance mode and measure the resistance between the loose ground and the neutral (Not the hot!!! If you mistakenly connect to a hot in resistance mode, you will at the least blow a fuse in the meter and might damage the meter.) A low resistance between the ground wire (~0.1 - 0.3 ohm) means the ground wire goes to the panel and is bonded to the neutral there. That is what you hope for.

If the resistance is higher (say 1 ohm to 500 ohm), then that would mean the wire goes to a ground rod or to a water pipe, but is not bonded to the neutral in the panel. I think this wire would still be usable as a ground for a GFCI receptacle, but expert opinion will be needed.

If the resistance is high kilo- or mega-ohms, that would mean the ground wire is not connected to anything and is useless as a ground.