Have you checked for small slugs and snails as a first line of enquiry? It looks like eelworm damage but eelworms on glads attack the bulbs rather than the leaves. I'd go for small slugs.

several bugs feed on gladiola, slugs, snails, grasshoppers, caterpillars, beetles. I don't think the copper band does anything to stop them so I won't suggest it. The best remedy is to go out at night with a flash light, bucket of water and a pair or thongs/tweezers. Pluck the critters off and drown them.