Commission status and information (Google Docs)
What is the heat gauge?
In MHGU the heat gauge was a gauge that would build up when using shelling, and go down when using physical attacks. The gauge went from yellow to orange to red, with yellow reducing physical damage and red giving a 5% damage boost. If you shelled past red, the gauge would reset to yellow and lock itself, preventing you from building up any heat or using wyvernfire. If you used wyvernfire, the gauge would lock itself at slightly lower gauge than you currently had, allowing you to lock your heat gauge at red.
In this mod the heat gauge still builds up from shelling and reduces heat when using physical attacks. However, the entire locking mechanic has been thrown away in favor of a black portion of the gauge. Yellow gives no boost, whilst red gives a large boost to shelling and physical. Black, on the other hand, gives a small damage reduction to shelling but the largest damage boost to physical attacks.

1.2.0 Update: New Balance Mode (currently in a testing phase)
Update adds a new toggle to the REFramework menu, which changes how the gauge acts.
Gauge now goes Yellow > Orange > Red > Orange > Yellow. This mode was added to encourage a more mixed playstyle, unlike the previous gauge which instead encouraged swapping between the two.

What are the numbers?
Damage changes
In MHGU:
While the actual percentages used were buffs, the gunlance's general damage output was reduced from MH4U:
[Color: Real percent (Percent relative to 4U)]
Yellow: 0% (-15%)
Orange: 15% (0%)
Red: 20% (5%) 
In this mod:
Yellow: 0% phys, 0% shelling, 0% Wyvern's Fire
Orange: 5% phys, 15% shelling, 10% Wyvern's Fire
Red: 10% phys, 25% shelling, 20% Wyvern's Fire
Black: 25% phys, 5% shelling, 30% Wyvern's Fire

Gauge length:
In this mod:
The gauge is a total of 150 units.
Yellow: 0 to 30
Orange: 30 to 80
Red: 80 to 125
Black: 125 to 150

Buildup:
In this mod:
Any spent shell: 5
Charging a shell: Gradually builds up heat. Heat buildup is increased when charging wide's bonus level. Note that charged shells will always use the damage of the gauge where the charge started. Charging from red into black will still do 25% extra shelling damage.
Ground splitter: Builds up heat towards the middle of the red gauge. Heat generation improves the farther away the gauge is from red.

Decay:
In this mod:
2 - Lateral Thrust
1 - Guard Thrust
10 - Lunging Upthrust
3 - Rising Slash
5 - Overhead Smash
7 - Wide Sweep

0 - Wyrmstake Plant

5 - Hail Cutter Jump
20 - Hail Cutter Landing
0 - Ground Splitter

5 - Jumping Thrust
10 - Jumping Smash
5 - Jumping Smash Landing

Wyvern's Fire:
Wyvern's fire consumes all the heat currently stored in the gunlance in order to improve its damage (No, this doesn't lock the gauge).

Artillery:
In this mod: Artillery increases the size of the red gauge and black gauge, and decreases the size of the yellow gauge. It additionally increases heat generation while outside of red, and decreases overall heat reduction. The specific numbers are:
[Artillery level: (Yellow gauge size, Orange gauge size, Red gauge size, Black gauge size), Buildup increase, Reduction decrease]
1: (20, 50, 50, 30), Increase: 10%, Decrease: 10%
2: (10, 50, 55, 35), Increase 30%, Decrease: 30%
3: (5, 45, 60, 40), Increase 50%, Decrease 50%

Customization:
All customization options can be found in the REFramework menu > Script Generated UI

• The mod can be toggled on and off
• There is a compatibility toggle for Powered Shelling
• The gauge can be moved around and resized.
  

Online:
The mod should work as long as you are the host. It will affect all players in the lobby, even if they don't have the mod installed.