In Baldur's Gate 3, the last-selected class during leveling up determines your spellcasting modifier attribute
Only for spells from items (Spells granted directly by items or scrolls)
Otherwise spells all use the spellcasting modifer of the class which acquired them.
This enables classes to swap into INT because of Wizard's ability to scribe any spells they have the spell slots for, regardless of their Wizard level (A level 10 Cleric with 1 Wizard level can scribe any and all Wizard spells in the game for example) giving them access to a full spell list that uses INT with a single dip.
It doesn't work any other way (Trying to make spells use CHR or WIS) due to the inability to acquire a full spell list of either stat without having invested the 11 levels in one of those classes (Emphasis on "One" you can't multi-class say Druid and Cleric and still get 6th level WIS spells)
It ONLY works by way of being able to obtain the full Wizard spell list (Which is more complete than what Sorc naturally gets) via scribing.
Who is your preferred melee caster: Gale or Wyll?
Wyll (Or rather, Warlock).
Hexblade = CHR for Weapon (Regardless of pact) and Medium Armour proficiency. While Warlock overall gets access to Agonizing Blast to get spell stat on Eldritch Blast. CHR base also means later you can opt to forgo Medium Armour for Potent Robe to get CHR mod on Cantrips (At like level 7-8. Well before the level 10 that Evo Wiz needs to get their INT mod on any spells) which works well with both Eldritch Blast and also Booming Blade (Booming Blade also benefits from Arcane Synergy allowing even more CHR on weapon damage rolls)
Gale (Or rather, Wizard) has some potential, but it's more for niche builds. Things like Abjurer Tank, Arcane Trickster Multi (To abuse Magical Ambush) or going Bladesinger for style (With its unique and somewhat flamboyant attack animations).
Technically, if we're going by character rather than class...
Gale could in fact conceivably be a Bard given its overlap with Wizard (Studying magic) and also his lore aspects (Preferring Abjuration, becoming a professor of Illusion). Wherein Sword Bard can contend with Warlock. More of a martial focus given needing STR/DEX for weapons (Though, nothing a Hexblade dip can't solve... While also granting Booming Blade and Eldritch Blast) and a plethora of spells, with a topping of Flourishes and their incredible propensity to be obscenely broken.
There isn't really a good melee damage spell for Wizards, like Inflict Wounds for Clerics.
Anything that doesn't use an Attack Roll is viable in melee range (I.e. Most AoE's like Burning Hands, Shatter, Lightning Bolt). Also Magic Missile which has no attack/save roll because it always hits (Unless Shield)
In addition, there's the Daredevil Gloves you can get in the Creche which allows you to make any ranged attack roll spells into Melee spells (Though, Wizard doesn't have that many Attack Roll spells anyway just... Ice Knife, Witch Bolt, Scorching Ray and Melf's Acid Arrow... Most of these kind of suck anyway with Scorching Ray being most relevant but only for Dragon Sorc whom gets their CHR mod on each bolt. Evo Wiz prefers just using Magic Missile/Art of War with their INT mod at level 10+)
