Aatrox W costs health. He has to choose between healing or damage.
Mundo Qs don’t heal, they refund on hit. His W costs a ton of health early on, that’s how it’s gated. He isn’t gated by these costs after level 6, when laning is over since his target access is pretty bad.
Gnar’s rage actually gates him. He can’t ranged harass as mega gnar (besides q which he’ll have to go pick up).
Morde has very real health costs. His W does not completely refund them at all (on minions).
Rek Sai isn’t endlessly harassing with heavily abusive abilities. Target access and risk is why this diver is manaless. Look at another diver, Jarvan, he can poke HEAVILY with Q and deny trade damage with W. Thus, mana gated.
Renekton. Long. Looong cooldowns.
Rengar, Riven, Kled, Shyvanna and Tryndamere are gated by target accessibility and/or heavy risk involved (no escape once spells are used to get in and they need to use them to get in) and/or cooldowns and/or target access. Fizz for example has mana because he can dash twice in a rotation (in and out).
Vlad has very real health costs as well. That W is hefty and his E’s cost is enough to make you use it sparingly (in lane anyway). Post-lane he is gated by his lack of mobility and short range.
Zac has to actively retrieve his blobs which isn’t easy in a laning or teamfight scenario, in which case every spell cast is dealing 4% current health damage to him.
Examples of why people have mana costs despite above mentioned drawbacks:
Udyr: obnoxious bear stance/turtle stance spam. Gates his mobility and his clear efficiency.
Olaf: those axes hurt. W grants insane healing, mana gates his jg sustain and clear speed.
Mages: long range, usually spammable, ranged waveclear. If it was infinite it’d be a mess.
Talon: W waveclear and poke gating. Q heal gating. Ult has a high cost due to how impactful it is which is compensated with a lower cooldown.
Master Yi: obnoxious laner if alpha and meditate had no mana costs. Already has high cooldowns as well, making them higher to compensate for manaless would be ridiculous.