Is ground (GND) possitive or negative?
Think of ground as the Earth beneath your feet.  That's where the name comes from.

It doesn't matter to the circuit whether the most positive node or the most negative node is connected to ground - ground is simply a reference point from which all of the other voltages are measured.

Note that Ground is also much more than a reference point - it is part of Electric Power Distribution safety and is important when dealing with sensitive, low-level signals.  But for the purposes of THIS discussion, let's just talk about it being the reference point.

Telecom systems world-wide use -48 Vdc (nominal) as the talk battery for their phone system.  That is: the live conductor on a telephone line is sitting at about -48 Vdc if you measure it using a multimeter with the (-) lead of the meter connected to Earth ground.

The reason for choosing to use -48 Vdc is physical - there are certain electrochemical reactions that occur if a telephone conductor should have a leakage path to ground.

Similarly, old automobiles (before the mid '60s) often used a -6 Vdc battery supply: the battery (+) terminal was tied to the chassis.

Electronic convention has current flow from (+) to (-).  The reasons for this are historic and even though we know now that this is incorrect, there is far too much existing infrastructure to consider changing that convention.