Add a new Rule to the Ruleset.  Call it “Experience” and give it the following text:

Each @ has a certain number of Experience Points, tracked in the GNDT under the column “XP”.  An @‘s Experience Points are initially 0.  Each @ also has an Experience Level, which is initially 1.  This may also be tracked in the same column.  If it is, the two values must be separated by a slash with the Experience Level first.  If an @‘s Experience Level isn’t tracked in the GNDT, it is assumed to be the smallest Experience Level with a Threshold greater than their XP.

An @‘s Experience Level is always at least 1 and at most 30.  Each possible Experience Level value has a Threshold associated with it.  The Threshold for Experience Level 1 is 20; the Threshold for Experience Level 10 is 10,000; The Threshold for Experience Level 20 is 10,000,000.  The Threshold for every other Experience Level is twice the Threshold of the previous Experience Level.  Whenever an @‘s Experience Points is greater than or equal to the Threshold for their Experience Level, they Gain a Level.

When an @ Gains a Level, roll DICE8, add 1 and increase their HP and Max HP by the result.  Their Experience Level increases by 1.  Then, if their Experience Points are greater than the Threshold for their new Experience Level, their Experience Points are set to one less than their new Experience Level’s Threshold.

Whenever an @ kills a Monster, each @ that Fought that monster and has not Died since then gains Experience Points equal to that Monster’s XP.

In the Rule entitled “Monsters”, change the text

each one has a number of HP and an amount of Damage, which is two integers (a number of Dice and a number of Sides).

to

each one has a number of HP and an amount of Damage, which is two integers (a number of Dice and a number of Sides).  A monster also has a number of XP. A Monster’s XP is always equal to that Monster’s initial HP, times that Monster’s Dice, times that Monster’s Sides, divided by three and rounded down, or 1 if the result is less than 1.