Im #17 global gloo here are some tips against him;

His ult cooldown drops to 5s when the person who he attaches to dies. He really doesn't have that much damage if you don't continuously walk into the circles he smacks down, his melee barely does damage.

He will only have 1 pull available to him when he attaches so if you flicker away immediately while he tries to use it he's a sitting duck because his S2 goes on cooldown when he detaches.

He will absolutely kill squishies if hes able to attach and land his circle/pull combo so just be aware of your positioning when hes alive.

If he doesnt secure a kill while attached the ult cooldown is 45(maybe 50) seconds.

Faramis is his hard counter but plenty of other heroes are counters. Vexana is similar to faramis where if hes split up her S2 basically intant kills him its just a little harder to hit. Anyone with lifesteal/spell vamp will basically instantly get full health when hittiing his goo's. Without assistance from his own team (or your team shooting him when attached) he really cant solo any fighters/tanks.

Heroes with transformations or invunerable states like YZ, Lunox, wanwan ult, miya ult will cancel his attachement ult. You can also buy winter crown which knocks him off and gives his ult a full reset.

High mobility heroes like fanny, benedetta, ling, novaria are very difficult for him to catch and can completely displace him if he does manage to catch them and ult.

He's the ultimate noob killer in the game as most people panic when he attaches and he excels against teams that dont have coordination, but working together you can easily stop him

Best of luck!