What makes up a gradient
In addition to other information others have written in comments, gradients measure the rate of change of "a quantity".

For example, take a hill. As you walk up the hill your elevation increases relative to the base of the hill. The steeper the hill the more quickly your elevation changes. The slope of the hill is defined as the gradient of the hill. The steeper the hill the greater the rate of change of elevation, with respect to the horizontal component of distance traveled.

With atmospheric gradients, imagine there are two towns, each with a weather station. The distance between the two is 100 km.

Each weather station measures pressure &amp; temperature at defined times, usually at half hourly intervals.

If the first town measures a pressure of 1011 hPa and temperature of 25 C @ 10 am and the second town, at 10 am, measures a pressure of 1008 hPa and a temperature of 20 C, then between the two towns there is a pressure gradient of 0.03 hPa/km [(1011-1008)/100]. Likewise there is a temperature gradient of 0.05 C/km [(25-20)/100].

Now, if at 11 am the weather station in the first town records a pressure of 1012.5 hPa and a temperature of 28 C, then over time, there has been a pressure gradient over the first town of 1.5 hPa/h [(1012.5-1011)/1] and a temperature gradient of 3 C/h [(28-25)/1].

So when it comes to gradients, it depends what is being measured (pressure, temperature, humidity) and what is it be measured against (distance, time, etc.), and for atmospheric quantities the distance could be lateral distance or vertical distance.