proper way to write the slang term for “gravitational force”

I came across something very similar to this in a thriller novel:

At this stage, the rocket is experiencing its maximum acceleration, say about ten gees.

Here, the author has spelled out the common military-pilot slang term for “gravitational force” using gee. However, as this term is an abbreviation, and is commonly treated as a unit of measurement (of acceleration) I'd have expected the written form to be g’s.

By analogy, I have heard people speak the unit “kilometer per hour” using the two-letter symbol for kilometer. However, I would expect this to be transcribed as “km per hour” versus “kay em per hour”.

Is this use of gee common? Is it correct?

This Google Books search shows that gees is a perfectly common way to write out the abbreviation for the force of gravity. Here are a few of the relevant citations:

• The outer four were all engineered to simulate gravity, in half-gee increments. Each layer consisted of three rings of ... The outermost triplet of rings was ten kilometres across and simulated gravity at two gees.
• The clamshells and nanite foam filling were in place, sealing us against the pressures, cushioning us against the growing acceleration—more than ten gees and building.
• We'd been boosting at half a gee, which isn't unpleasant. Two Mars gees would actually be three-quarters of an Earth gravity. Two-thirds Earth gees would be heavier than I like, but not nearly as onerous as Pismo Beach.
• The idea that life might exist under the extremely rigorous conditions scientists predict on the surface of a neutron star — surface gravity 1011 gees, surface temperature 104 kelvin, and equatorial magnetic field strength 108 tesla — was first ...
• Forward's novel, on the other hand, is of intelligent life on a neutron star with a radius of 10 kilometers and a surface gravity of 67 billion gees.
• ... field and being in an equivalent accelerated frame of—it means that when the Anniversary is blasting five gees, the effect on us is the same as if it were sitting on its tail on a big planet, on one with five gees' surface gravity.”
• One day the control shorted out in the rec. room and plastered the guests into the couch at 3 gees.
• The gravity on the floor of level one was almost four and a half gees.
• If there is no bounce and little room in which to decelerate, the magnitude of gee is so high as to be deadly. The force on an object due to Earth's gravity near Earth is equal to the mass of the body multiplied by 9.8 m/s2 (1 gee) and
• number ot gees =1.0 32 ft/sec2 b F = 1.6 (1 lb) = 1.6 lb In Example 2-7 we introduced the term "gees" of gravity. When we state that a jet pilot experienced 4 "gees" we mean he felt four times the pull of gravity due to an acceleration or ...
• And what we know today about gee forces proves that Ar-dan's dear adversary is absolutely right. A gee is a unit for measuring how rapidly a body's speed changes. One gee is the acceleration caused by gravity as a body falls near the ...
• Then he felt gravity again as some force began to slow him down. The chair spun around so that Hackworth was looking up into the irregular constellation of chandeliers, and the acceleration shot up to several gees. Then back to normal.

The usual way to denote an acceleration of "ten gee" in technical writing would be 10g (lower case 'g'). Upper case 'G' usually denotes the Universal Gravitational Constant that appears in the law of gravitational attraction.