It was mostly likely holding the Heavy-Duty Boots
Heavy-Duty Boots (Japanese: あつぞこブーツ Thick-Soled Boots) is a type of held item introduced in Generation VIII.
The holder is unaffected by the effects of entry hazards.
Source: https://bulbapedia.bulbagarden.net/wiki/Heavy-Duty_Boots
(Entry hazards = Spikes, Pointed stones (Stealth Rock), Sticky Web, Poison spikes (Toxic Spikes) and Sharp steel (G-Max Steelsurge))
A lot Pokémon hold this item in competitive battles to avoid getting crippled by hazards (especially Stealth Rock). In this case Galvantula was holding the boots to avoid losing 1/4 of its health thanks to the battle staple pointed stones, so it wasn't slowed down by your Sticky Web.
