Gourry lost his Sword of Light in the end of TRT. Did he find the new sword in the novels?
Yes, he did. He found a magical sword in the 9th book of Slayers novels. At first, both Lina and Gourry didn't know what the sword was, but during the battle against Dynast Grawsheller, the Mazoku held the blade of Gourry's sword, revealing that the sword has another blade within its blade. Mirgazia, the eldest gold dragon from Dragon's Peak knew that the sword was legendary Blast Sword. When Lina and Gourry confronted with the battle against another resurrected piece of Ruby Eye, the dark lord told them that Blast Sword changed the magical power around it into its sharpness.
Laavas shot magical blade again and again, cutting apart the stone floor, all the magic items, all the things in this treasure room...
No!
Almost every thing were torn apart by Laavas' blades of magical power, but among them, there was something, glittering silver.
It was the blade of a sword, shining through its torn sheath, wasn't it?
"Gourry!! Take that sword!"
"Yes!"
Gourry rushed to the sword.
Laavas shot the blade of magical power at Gourry!
Oh! Gourry couldn't dodge it?
ZANK!
It was the magiacl blade of Laavas that was cut apart.
Gourry had cut the magical blade with the sword in his hands.
(...)
"Oh?"
Laavas seemed to became anxious.
He jumped back to the wall and took another magical sword, unsheathing it.
Gourry attacked Laavas without hesitation.
Laavas held the sword in his right hand, shooting the magical blades with the left hand again and again.
But, every blades were cut or knocked down by Gourry's sword!
(from the tenth book, "Trick in Solaria")
(Lina, Gourry, Luke, Milina, Mirgazia and Menphis were fighting against Dynast Grawsheller.)
"Ruby Eye Blade!" Luke shouted.
It was the second time for him to use that spell, swinging the magical sword to Dynast.
"No, you can't!"
Dynast charged his magical power to his bastard sword and protected himself. But he couldn't move as fast as before.
And from the other side...
"Wooooooooo!"
Gourry swang his sword.
But, Dynast didn't care about Gourry. Even if the Mazoku became half-powerful, or less than harf-powerful, Gourry's sword could give a little damage to Dynast.
At the instant when Gourry's sword was going to hit Dynast's body!
Wowooooooooooo!!
Mirgaria roared, and at the same time, the red glittering pattern appeared on the blade of Gourry's sword!
--- Oh, yes! Gourry told me that Mirgazia drew some patterns on the blade of his sword ---
--- I see! Mirgazia drew that patterns with the blood of a dragon... his blood!
The pattersn on the balde resonated with Mirgazia's power, and the blade stuck to Dynast's body!
"Gwaaa!"
Dynast gave a low cry.
Yes, it worked a little!
Grawsheller swang his left hand and grabbed the blade without hesitation!
Ping!
The low noise of the cracking blade echoed.
"You, small knife!"
CRACK!
Shouting, Dynast crushed the blade in his hand!
--- Then ---
It appeared.
Another thin blade appeared from the broken blade!
"What?!"
"?!"
All of us were surprised to see it ---
"Blast Sword?"
Mirgazia said, and at the same time, Gourry stuck the lavender-glittering blade deep into Dynast's side!
"Gwaooooooo!!"
Grawsheller gave a cry of pain...
(...)
--- Legendary sword, Blast Sword...
Why the true blade had been hidden in another blade?
Soon, we learned the reason.
It was extremly sharp. The sharpness of the blade was the top level --- as extreme as to be useless.
When we dropped the sword with the point down, the blade first stuck to the stone floor, then fell down, cutting the stones apart. If we sheathed the blade and swang the sword, the sheath would be cut apart.
Moreover, if the sheath was made of the wood or leather, it was bisected when someone sheathed the blade.
... So, this sword was too dangerous to bring...
If I brought Blast Sword, walking on the street, and if I strumbled over something and fell on the ground, the blade would break the sheath apart, and cut everything it touched.
How could a man bring such a dangerous sword? It was just an automatic killing machine.
I knew that this blade was nothing but a silly item with extreme sharpness.
Probably, someone had thought the same before, and he covered the sharp blade with another blade, that had been made of a very strong steel.
Meaningless.... It was just meaningless...
Now, Gourry could bring the sword, because Mirgazia drew another pattern on the blade, making the blade a little dull.
It became dull a little, still Gourry could cut a rock with it without his full power.
(from the thirteenth book, "The Omen of Drak Lord's Descent")
(Lina and Gourry were trapped in another space where they confronted with the battle against the resurrected piece of Shabranigudo.)
"Here ---" the red Death said.
"Here is the space that is filled with the magical power; 'Maryoku'.
'Ma' is the power that doesn't exist in the world naturally.
This world stands a skin outside of the original universe, so, the border from the other universes are thinner.
Thus, the air is filled with Maryoku here.
You, two, already felt it.
The magical energy in this space."
... I see... I have been feeling something, and that something must be the thickness of the mgaicl power.
The red Death continued.
"In this space---
Girl, the blade of darkness that you make won't disappear in a few seconds.
You don't need to speak the incantation, just need your power of will and 'the word with power' to use a spell.
Man, your sword absorbs the magical power around it and make the blade sharper. So, your sword will be ablde to give a damage to me.
I mean to say ---
you, two, have the ability to destroy me."
(from the fifteenth book, "The Demon Slayers")