Firstly your force is highly under-commanded. Gilbert and your division commander (McArthur?) give 4 CP's (Command Points), but the division needs 4 CP's. The other 2 brigades, the militia regiment, artillery battery and anything that might be below them add to the CP need. Unfortunately the Battle Plan doesn't show this.
The bde's probably need 3 CP each, 1 for the mil. reg. and 1 for the bty. That makes, with the division, 12 CP needed for the entire force, but Gilbert and what's-his-face offering only 4 ((12CP-needed - 4CP-give) / 4CP-given) * 100 = 200% : your force is 200% under-commanded.
When more than 35% under-commanded your force doesn't take any further penalty, just(!!) -35% loss in strength.
The second major issue is that Gilbert is just a brigadier general, while A. S. Johnston is a Lt. Gen.. Not only are there some minor bonuses per command level above a brigade or division (if for example Forney is a corp commander), but Johnston at any rate will give the force under him about a 5% bonus for being 1 level above Hardee and Cheatham. If Forney were corp commander he'd give another 3%.
But the biggest issue is frontage. A Lt. Gen. will provide much higher frontage than a Brig. Gen. Without going into the hard details ('couse I'd have to look them up myself and I'm a lazy bastard today :neener

the higher ranking your force commander is, the more elements he can put into the same length of the front, the frontage. So on your side Gilbert might put 2 regiments per 100 yards of the front, while Johnston might squeeze 4 regiments in the same section. So in that section your 2 rgt. will be getting hit by 4 rgt., plus artillery.
If in the mountains, forest or swamp, this might not be so bad, because the total length of the front is much shorter. But in the open ground around Lexington the length of the front can grow to it's maximum, and your front line will be over-lapped, which is very bad for your force.
Then factor in Johnston's Offensive Value of 2, plus Hardee's and Cheatham's vs. Gilbert's 1 defensive and the whole thing looks pretty bad for your defenses in Lexington, even with your good entrenchments. BTW the 2 bde's, the mil., and the bty. were all not in entrenchments.