Taking on Slave Knight Gael with purely sorceries is a tough battle but can be done if you keep your distance. Alot of his attacks will outright miss if you run away from him and remember which attacks where you have to roll to the side instead.
Being able to defeat Gael also requires FP management and avoiding most of his attacks as you need alot of Ashen estus for this fight. Running out of FP during the fight is possible and I have died 1-2 hits away just because I didn't have enough FP to finish Gael off. I suggest at least 7 Ashen Estus. Simple infused weapons could be helpful but it takes way too long just to recover enough FP for one CSS casting in my opinion.
With a +10 Court Sorcerer's staff, the Scholar's Candlestick, Crown of Dusk, Bellowing Dragoncrest Ring, Young Dragon Ring, and Sage Ring +2 at 60 INT and 28 Attunement, you'll have 4 Crystal Soul Spear (CSS) casts and 2 Heavy Great Soul Arrows (HGSA) before running out of FP and Ashen Estus.
When you first enter the arena, Gael will be very far away but rushing to your position. You can get two CSS castings in with the Sage Ring +2 before having to dodge his first attack.
Roll to the right, and then punish immediately with CSS.
Run away and roll back before he plunges down with his sword and then punish with CSS.
Run away and roll back if you're too close right before he initiates each attack. Punish after his third attack.
Run away and roll to the right before each attack. Punish right after his 2nd attack.
Phase two will occur roughly after taking 1/3 of his HP. I suggest recovering your FP to max before the your last hit that will initiate the 2nd phase. This way you can get 2, possibly 3 free CSSs and stagger Gael if he attacks.
-Repeating Crossbow WA
Stay at medium distance and circle rightwards Gael. Punish after he uses it twice in a row.
-Way of the White Corona
Circling Gael rightwards and running away will avoid the attack when the disks come boomeranging back toward him. Gael usually uses his repeating Crossbow right after so punish right after his crossbow animation ends and you're safely out of range of the disks.
If Gael rushes toward you and try to attack you up close, rolling to the right is much better than rolling back as his coat will extend the range of his sword attack.
-Soul Explosion (1/3 HP Bar left)
Get as far away as possible but in range of CSS. You can get two castings of Crystal Soul Spear with the Sage Ring +2 before having to dodge his souls. It's free damage so try to restore your FP right before you take out 1/3 of his HP.
Stay as far away as you can and roll right. He has two attack sequences he'll primarily use as well as the previous attacks he has.
-One where he's charged with souls and does 3 attacks in a row. You can punish Gael with a CSS if you're far away enough to avoid his tracking souls after his 3rd attack.
-The other attack sequence, Gael does multiple flips in the air. Run away and roll. Punish with CSS after his 2nd attack.
I suggest only attacking after he uses his two-attack sequence where he spins in the air. It's the safest option by far compared to his soul-charging sword attack.