Question

What units are used to measure gas pressure $?$

Answer 1

First we have to know what gas pressure is. How pressure relates with other physical properties. Then discuss the SI units of gas pressure. Give some devices we use to measure gas pressure.
Complete answer:
Gas pressure is caused when gas particles hit the walls of their container. The more often the particles hit the walls, and the faster they are moving when they do this, the higher the pressure.
The SI unit of pressure is the pascal ($Pa$), with $1\;pa = 1\;N/{m^2}$, where $N$ is the newton, a unit of force defined as $1\;kg\;m/{s^2}$. One pascal is a small pressure; in many cases, it is more convenient to use units of kilopascal ($1\;kPa = 1000\;Pa$) or bar ($1\;bar = 100000\;Pa$). In the United States, pressure is often measured in pounds of force on an area of one square inch [pounds per square inch ($psi$)]. Pressure can also be measured using the unit atmosphere ($atm$), which originally represented the average sea level air pressure at the approximate latitude of Paris (${45^0}$).
Some common units for pressure measurements

Unit Name and Abbreviation	Definition or Relation to Other Unit
pascal ($Pa$)	$1\;pa = 1\;N/{m^2}$
kilopascal ($kPa$)	$1\;kPa = 1000\;Pa$
pounds per square inch ($psi$)	air pressure at sea level is $\sim 14.7\;psi$
atmosphere ($atm$)	$1\;atm = 101325\;Pa$
$bar$	$1\;bar = 100000\;Pa$
millibar (mbar, or mb)	$1000\;mbar = 1\;bar$
inches of mercury (in. Hg)	1 in. Hg = 3386 Pa
Torr	$1\;torr = 1760\;atm$
millimeters of mercury ($mm\;Hg$)	$1\;mm\;Hg \sim 1\;torr$

Note:
Note that the devices we use to measure gas pressure are PRESSURE GAUGE is measuring pressures and typical pressure ranges go from $0\;bar$ up to $600\;bar$. ANEROID BAROMETER is a pressure gauge that is calibrated to measure pressures in the approximate range from $950$ to $1070\;mbar$. U-TUBE MANOMETER contains water or mercury in a U-shaped tube. One end of the U tube is exposed to the unknown pressure, and the other end is open to the atmosphere. VACUUM GAUGE is similar to a pressure gauge. Common gauges measure pressures from $1000\;mbar$ to about $50\;mbar$. McLEOD GAUGE isolates a sample of gas and compresses it in a modified mercury manometer until the pressure is a few millibars. Its useful range is from $100\;nbar$ to $1\;nbar$.