Hellcat gauntlets(Magic Item Compendium pg.111) can be activated to add 1d6 points of slashing damage per spell level to the next spell cast. Can they be used with the Warlocks Eldritch Blast spell-like ability?
No, Eldritch Blast is a Spell-Like Ability, not a Spell.
As a DM, what would I do? I would say, yes it does.
Warlocks are not CoDzillas and can typically use all the help they can get. I would base my ruling on the information provided below:
An eldritch blast is the equivalent of a 1st-level spell. If you apply a blast shape or eldritch essence invocation to your eldritch blast (see page 130), your eldritch blast uses the level equivalent of the shape or essence.
Metamagic feats cannot improve a warlock's eldritch blast (because it is a spell-like ability, not a spell).
When you activate hellcat gauntlets, the next spell you cast during your turn that targets a single creature also deals 1d6 points of slashing damage per level of the spell.
Since Eldritch Blast is equivalent to a spell, I would allow it to be used with Hellcat Gauntlets.
In the Name of Fluff
Warlocks, typically, are fiendish in nature, and particularly affiliated with fiends of the Nine Hells (devils). Warlocks, especially if using the prestige class that allows them to use Hellfire Blast, relish in the powers of Hell, and Hellcat Gauntlets would improve upon that namesake.