Here's a table which shows the amount of damage caused by a gas pocket at a given depth and radiator. (Rounded to nearest integer)

Depth Stock Lv1 Lv2 Lv3 Lv4 Lv5
5000 -133 -106 -100 -80 -53 -26
5100 -140 -112 -105 -84 -56 -28
5200 -146 -117 -110 -87 -58 -29
5300 -153 -122 -115 -92 -61 -30
5400 -160 -128 -120 -96 -64 -32
5500 -166 -133 -125 -99 -66 -33
5600 -173 -138 -130 -104 -69 -34
5700 -180 -144 -135 -108 -72 -36
5800 -186 -149 -140 -111 -74 -37
5900 -193 -154 -145 -116 -77 -38
6000 -200 -160 -150 -120 -80 -40
6100 -206 -165 -155 -123 -82 -41
6200 -213 -170 -160 -128 -85 -42
6300 -220 -176 -165 -132 -88 -44
6400 -226 -181 -170 -136 -90 -45
6500 -233 -186 -175 -140 -93 -46
6600 -240 -192 -180 -144 -96 -48
6700 -246 -197 -185 -148 -98 -49
6800 -253 -202 -190 -152 -101 -50
6900 -260 -208 -195 -156 -104 -52
7000 -266 -213 -200 -160 -106 -53
7100 -273 -218 -205 -163 -109 -54
7200 -280 -224 -210 -168 -112 -56
7300 -286 -229 -215 -172 -114 -57

For further details, here is the relevant code.

function hitGasPocket()
{
atv.damage(int((- depth + 3000) / 15) * atv.radiatorCooling[atv.radiator]);
_root.attachMovie("gasPocketFX","gasPocketFX",9988776656);
}

this.radiatorCooling = new Array(1,0.9,0.75,0.6,0.4,0.2);

Any word on how hitGasPocket() is called?

hitGasPocket() gets called by the ATV sprite whenever a gas pocket block gets dug out.