I think Slowbro is a fairly decent counter. It has the bulk to take Gliscors moves, especially with Regenerator and Slack off. Gliscor only has base 95 attack, which isn't all that much when you consider it pretty much never runs choice band or life orb. Slowbro also has Ice Beam to destroy Gliscor with.
Slowbro can't really deal with ToxicSub Stall Gliscor. If Slowbro gets poisoned on the switch, its awful speed means that between Sub and Protect, Slowbro will never be hitting Gliscor directly, and proceed to have to either switch out or get stalled to death. Barring Trick Room intervention, of course.
Counters
Gliscor's one of those Pokemon whose counters really depend on what moves its running.
Skarmory - Gliscor can't really do anything to Skarmory, as its immune to Toxic and Earthquake, takes pittance from Acrobatics and Ice Fang, and can just set up hazards or whirlwind Gliscor away to get rid of its Sub and any boosts. However, if Gliscor has Taunt, then Skarmory won't be able to do anything to it, either, as Brave Bird won't do much to the physically bulky Gliscor.
Gengar - Gengar laughs at Sub Stalling sets, as it's immune to Toxic and Earthquake, and isn't crippled by Taunt as it can just pound away at Gliscor's poor Special Defense. However, Gengar is frail, so it can get shredded by Acrobatics and doesn't like eating Ice Fangs or Stone Edges either.
Rotom-W - Rotom-W deals with offensive sets without difficulty, as its immune to Earthquake, resists both Ice Fang and Acrobatics, can usually take a Stone Edge, and can 1HKO with either Hydro Pump or HP Ice. In addition, many Rotom-W's carry Will-o-Wisp, and if they can get that off before Toxic Orb activates, Gliscor will be crippled (there is a very small window of opportunity to do this, though). However, Rotom-W is susceptible to Toxic Stalling, so it has to be wary of what set Gliscor is running when it switches in.
Cloyster - Cloyster makes a decent check, as it is not 2HKO'd by anything barring the odd Stone Edge (and hitting 2 Stone Edges in a row can sometimes be asking for a lot) due to its physical bulk, and Icicle Spear + Skill Link has no trouble killing Gliscor even if it's behind a Sub. It doesn't like eating Toxics though, and boosted attacks can potentially be a bit much for it to handle.
Mamomswine - Mamoswine is largely in the same boat as Cloyster, though without Skill Link, its Icicle Spears are far less reliable. Mamoswine's Ice Shards can be a headache for Gliscor, but with a Sub up, Mamoswine can run into trouble with that and be forced to eat Toxic.
Sabeleye - Sableye can be an incredibly situational counter. Like Rotom-W, it can try to burn Gliscor with Will-o-Wisp (aided by Prankster) to circumvent Poison Heal, but there's only a single turn to do this. Likewise, Sableye's Prankstered Taunts can prevent Gliscor from stalling at all. Sableye's pretty frail and can't eat boosted attacks though, but it can Recover stall Gliscor's PP away on unboosted attacks.
Faster Pokemon with good special attack can revenge kill Gliscor without much difficulty, especially if it's not behind a sub. Things like Keldeo, Starmie, and Jolteon carrying HP Ice can all prey on Gliscor's poor special defense, but many of them have difficulty switching in. Things like Latios, Latias, and Thundurus-T (again with HP Ice) are great at checking Gliscor as well, as they can often nab a free switch in if you predict an Earthquake. However, none of them like Toxic or Ice Fang, so they have to be careful on what they switch into.
And of course, faster Pokemon with Taunt can shut down Stall Gliscors, forcing them out. SubToxic Gliscor in particular is very vulnerable to taunt, as it usually carried only one offensive move, which is usually Earthquake.