What is the basic difference between absolute pressure and gauge pressure?
According to Wikipedia :
Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."
So suppose you are in an ocean at height depth,your absolute pressure would be $Patm+\rho*g*d$ while your guage pressure would be simply $\rho*g*d$, also you can say if at any point your total pressure is P your absolute pressure is $P$ while your guage pressure is $P - Patm$