Are you saying that the G2 tail cap wouldnt work?, or wouldt it work more like the A2 Aviator, where i could just push through the lower light to the higher light
Any of the G/P/C tailcaps will work on any of the G/P/C lights. They are interchangeable. However, they do not all work the same. The standard G/P/C tailcaps are press-for-momentary/twist-for-constant. So a press of the switch turns the light on, and keeps it on as long as you hold pressure on it. Release pressure, and the next time you press the switch, it will turn on again. In the case of your G2X Pro, a press-release-press within a couple seconds with one of these standard G/P/C tailcaps will cycle the light from low to high and back to low. To turn the light on permanently, you twist the tailcap clockwise. A quick twist-untwist-twist will cycle the light just like the press-release-press. But no tailcap will change the fact that your G2X Pro is a dual output light. The programming is on the circuit board glued inside the head of the light.
The tailcap on the A2 Aviator (which can also be found on the LX2) is Surefire's most sophisticated tail switch. It is also my favorite design, as I like pressing or twisting for low, then pressing harder for high. Unfortunately, not only does Surefire not sell this tailcap separately, but it would not fit on your G2X Pro. No tailcap that will fit the G/P/C lights works this way.
Your best (although not the easiest) option is to locate a used or deeply discounted C2 Centurion (these have been discontinued), and pull out the stock P60 incandescent lamp assembly, and drop in a Malkoff M61. This combination can be put together for around $100, and cannot be beat for durability and quality (for the price).
Surefire also still sells the 6P Original and the G2 (which can be modified with aftermarket drop-ins). However, the G2's plastic construction and lens make it impractical for long-term use with a very bright lamp or LED. The aluminum 6P Original with glass lens can withstand hundreds of lumens for long periods of time. It's design is now about 20 years old, but it's still the gold standard.
The easiest solution would be to just buy a Surefire "Tactical" model such as the 6PX Tactical, G2X Tactical, G2Z, G2ZX, or Z2X. All of these models will cost at least $20 more than you paid for your G2X Pro.
Surefire's new lights have sealed heads and bezels, and cannot be upgraded or modified. You get what you get. Their older models (6P Original and Defender, 9P, G2 Original, C2, C3, etc.) all use the same head/bezel design that can just be twisted off the body. The lamp or LED module can be removed, and any number of aftermarket modules can just be dropped right in. In the flashaholic world, this is referred to as Surefire "Lego'ing". Much like the AR-15, these older Surefire models can be configured in dozens of different ways to suit the user. There is much disappointment in the flashaholic community over Surefire's decision to seal up all these components in their newer models so the lights cannot be modified. Boo!