I'm fairly new to CG, and also have 2C hair
The last time I co-washed, I had the exact same experience. I'd used a lot of gel over the course of the previous few days to revive my hair between washes, plus I always use post-shower leave-ins, although up until now I haven't been adding an additional rinse-out after co-washing. (I wash/co-wash only every four or five days because of the dryness and porosity)
I had this really gross sensation in the shower when I first got my hair wet of scads of gel liquefying and running down my neck and back all at once. GROSS! The conditioner seemed to cleanse my scalp OK, but it felt like I never got all the gel out, and the strands all seemed coated afterwards.
I still did the post-shower leave-in and detangle, and then used a lighter gel this time. My hair has been a lot more manageable this week, although the waves are looser and I'm afraid to put more gel in it at this point, because I think that while it may perk up my waves (good), it will just cause more gross build-up (bad).
I think wavies like us need to use low-poos a lot more frequently than curlies do. At this point, I'll co-wash if it's only been a day or so since my last wash or if I've used very little product since my last wash.
(Or *guilty look*) I'll co-wash instead of low-poo if I want my waves to look looser/straighter, which I've noticed I can do without heat now if I comb/rake my gels/conditioners/stylers through my hair instead of scrunching and comb my hair every morning and night whether it's wet or not - kind of a traitor thought if seen as an affront to the "embrace your natural texture at all times" philosophy, but I like the versatility I can get without having to use damaging heat appliances. I never make it "straight-straight" this way, because I don't want that,but I can loosen the waves down to a 2b-ish level if I continue to use CG products, co-wash instead of low-poo and comb rather than scrunch. Versatility rocks.
After a couple weeks of this, though, I sometimes have to cave and use a sulfate shampoo without silicones because I end up with buildup - I have hard water and fine wavy hair and all the mineral deposits and hair products overwhelm my fine strands completely. I just have to make sure to follow up the 'poo with a DT/very thick and rich conditioner so I don't get fluff city with no curl pattern.