Boro looked to be staring their second defeat of the festive season straight in the face after two Michael Kightly goals had given the hosts what should have been an unassailable lead.
But, for the second game in a row, Grays' defending let them down and after Dino Maamria had halved the arrears with 12 minutes to go, Louis completed the comeback with a towering header at the back post.
Grays should have been in front inside the first 30 seconds when a free-flowing move involving Glenn Poole and the fit-again Tony Battersby ended in John Martin firing agonisingly wide from no more than 10 yards.
The hosts broke the deadlock midway through the half when, after seeing out a spell of Boro pressure, Martin lofted a long ball towards the speedy Kightly who shrugged off the attentions of Michael Warner, skipped past keeper, Alan Julian and rolled the ball into the empty net.
Martin went desperately close again for Blues shortly after the re-start when Gary Hooper and Kightly combined to set up the midfielder but he shuddered the crossbar when he really should have scored.
Grays got the second when a half-cleared corner came to Martin at the back post and he pulled it back to Kightly who shot low into the bottom corner.
Martin then had his third great chance of the match when he met Kightly's cross but his header was repelled by a brilliant Julian stop.
Maamria started the fightback when he capitalised on a Jamie Stuart mistake to fire past Nicky Eyre and Louis restored parity with a great header from Warner's cross.