U.S. Military Ranks

GURPS
Rank
Alternate
Cost
Pay
Grade
Air Force Army Coast Guard Marine Corps Navy
9[1] 45 O-11[1]   General of the Armies[1] [1] [1] Admiral of the Navy[1]
General of the Air Force General of the Army Fleet Admiral
8 42 O-10 General (Air Force Chief of Staff) General Admiral (Commandant of the Coast Guard) General Admiral (Chief of Naval Operations)
40 O-9 Lieutenant General Lieutenant General Vice Admiral Lieutenant General Vice Admiral
7 37 O-8 Major General Major General Rear Admiral (Upper Half) Major General Rear Admiral (Upper Half)
35 O-7 Brigadier General Brigadier General Rear Admiral (Lower Half) Brigadier General Rear Admiral (Lower Half)
6 30 O-6 Colonel Colonel Captain Colonel Captain
5 25 O-5 Lieutenant Colonel Lieutenant Colonel Commander Lieutenant Colonel Commander
4 22 O-4 Major Major Lieutenant Commander Major Lieutenant Commander
20 O-3 Captain Captain Lieutenant Captain Lieutenant
3 17 O-2 First Lieutenant First Lieutenant Lieutenant Junior Grade First Lieutenant Lieutenant Junior Grade
15 O-1 Second Lieutenant Second Lieutenant Ensign Second Lieutenant Ensign
4[6] 22 W-5 [2] Chief Warrant Officer 5 N/A Chief Warrant Officer 5 Chief Warrant Officer 5
20 W-4 Chief Warrant Officer 4 Chief Warrant Officer 4 Chief Warrant Officer 4 Chief Warrant Officer 4
3 18 W-3 Chief Warrant Officer 3 Chief Warrant Officer 3 Chief Warrant Officer 3 Chief Warrant Officer 3
16 W-2 Chief Warrant Officer 2 Chief Warrant Officer 2 Chief Warrant Officer 2 Chief Warrant Officer 2
15 W-1 Warrant Officer 1 Warrant Officer 1[3] Warrant Officer 1[3] Warrant Officer 1[3]
4[6] 20 E-9[4] Chief Master Sergeant of the Air Force Sergeant Major of the Army Master Chief Petty Officer of the Coast Guard Sergeant Major of the Marine Corps Master Chief Petty Officer of the Navy
3 17 E-9 Command Chief Master Sergeant Command Sergeant Major Command Master Chief Petty Officer Sergeant Major Command Master Chief Petty Officer
15 E-9 Chief Master Sergeant[5] First Sergeant Master Chief Petty Officer Mastery Gunnery Sergeant Master Chief Petty Officer
2 12 E-8 Senior Master Sergeant[5] Master Sergeant Senior Chief Petty Officer First Sergeant Senior Chief Petty Officer
Master Sergeant
10 E-7 Master Sergeant[5] Sergeant First Class Chief Petty Officer Gunnery Sergeant Chief Petty Officer
1 7 E-6 Technical Sergeant Staff Sergeant Petty Officer 1st Class Staff Sergeant Petty Officer 1st Class
5 E-5 Staff Sergeant Sergeant Petty Officer 2nd Class Sergeant Petty Officer 2nd Class
0 3 E-4 Senior Airman Corporal Petty Officer 3rd Class Corporal Petty Officer 3rd Class
Specialist
2 E-3 Airman First Class Private 1st Class Airman
Fireman
Seaman
Lance Corporal Seaman
1 E-2 Airman Private 2 Seaman Apprentice
Fireman Apprentice
Seaman Apprentice
Private 1st Class Seaman Apprentice
0 E-1 Airman Basic Private Seaman Recruit Private Seaman Recruit

[1] O-11 grades are reserved for wartime and as such are fairly rare appointments. Since the Marine Corps and Coast Guard fall under Naval command they have no O-11 grade. You may want to treat O-11 as Rank 8 or as Rank 9, depending on your personal preference. Rank 9 is somewhat appropriate, though, as it's a grade that can only be earned in wartime so has a certain prestige to it.
    General of the Armies and Admiral of the Navy are special ranks, collectively held by only three men - Gen. John J. Pershing was awarded General of the Armies in 1919 for his service in WWI; Gen. George Washington was posthumously awarded General of the Armies by Congress in the 1976 U.S. Bicentennial celebration for his service during the Revolutionary War; and Adm. George Dewey was awarded Admiral of the Navy in 1903, retroactive to 1899, for his victory in Manila Bay in 1898 during the Spanish-American War.
[2] The Air Force stopped appointing Warrant Officers in 1959. While a small number existed for the next 21 years, and the ranks still technically exist today, they are no longer in use.
[3] Warrant Officer 1 was discontinued in the Navy and Marine Corps in 1975. While it still exists in the Coast Guard it's no longer used.
[4] There is only one E-9 of this pay grade for each service, representing the highest ranking NCO in their respective services.
[5] The Air Force maintains two ranks at each pay grade E-7 to E-9 with the same title but different rank insignias and different pay scales.
[6] A case can be made that the highest E-9 and W-4 to W-5 grades should be GURPS Rank 3 or 4, though technically they may both be only Rank 3. Even so, the highest E-9 ranked NCO reports directly to the O-10 Admiral/General of his service, which is pull that even O-7 grade officers do not have.

Alternate Cost

The alternate cost represents a system of "partial rank" to fill in the gaps where the regular Rank advantage doesn't cover. These partial ranks include some extra responsibility, and extra pay, so have a slight bonus to them. They would also give a +1 reaction bonus from anyone of the same GURPS Rank who serves under you.