Confusion regarding Fixed-Biased configuration of JFET
Shouldn't they be of the same polarity according to the connected
  terminals ?


No, the minus (-) terminal of the battery is indeed connected to the plus (+) side of \$V_{GS}\$, that is, the JFET gate (G).

All things considered, it's just an intrincate way of saying that \$V_{GS}\$ must be a negative voltage, i.e. the gate must have a negative bias voltage applied (when source is grounded).

Note: \$V_{GS}\$ means "Gate (G) to Source (S) Voltage". Voltage is a potential difference, and by definition that difference is \$V_G\$ ("positive" terminal) minus \$V_S\$ ("negative" terminal). However, such a difference can be negative (as it is in this case) because it's just a reference, a convention. The + and - signs doesn't tell you anything about which terminal is actually at a higher potential, just how the potential difference (voltage) has to be calculated.