"Grit" vs "gritted"

Why does that sound weird to you?

Because there is a small group of irregular monosyllabic English verbs ending in -t (e.g, hit, spit, shit, knit, fit, pet, let) that share the peculiarity of having Zero as their past tense and past participle form, too. So it's

• Present: He lets her in. She hits him in the face.
• Past: He let her in. She hit him in the face.
• Perfect He has let her in. She has hit him in the face.

Now, this is a small class, and the verbs in it are pretty common -- this is how they stay irregular, of course; verbs that aren't often encountered rapidly become regular. And with a verb like grit, there's a natural tendency to conjugate it like hit, spit, knit, fit; but grit is rare enough that maybe it ought to be regular. One can't know everything, after all; so pick whatever sounds good to you and stick with it.

I was surprised to hear petted, too; but I don't know how recent it is. Note that it's transitive, though. One thing to watch out for is differential regularity -- transitive usages, especially causative transitive usages, of irregular verbs may well become regular independently of the irregular corresponding intransitive.

• He shined his shoes ~ Those shoes shone brilliantly -- but not the reverse.