What is written here on page 4, after 「それと一緒に」? I searched in WWWJDIC and Google, but I can't understand the meaning of 「こつぐってなる」. For the 「語彙が…」 comment, I understand that this word sounds strange for the characters too. But strange is different from nonsense.
You misread one hiragana. It says
(So, not つ but う.)
The comment 「語彙が…」 refers to the fact that the girl is using a lot of onomatopoeic expressions (instead of proper vocabulary) to describe the first bite of the cake.
Here こう (sibling of そう、ああ、どう) means "like this". ぐってなる can also be written ぐっとなる and refers to some tight/cuddly/... feeling.
This fluffy white thing is in the mouth like *phwaaah*
Blending together with this *uhnnn* and going *mmm*
Not an award winning translation, but I trust you get the point =)
You've misparsed the phrase. It's こう + ぐって + なる. The う in this font looks very close to the つ, but note the dash on top.
As for what the ぐって is supposed to mean, I'm not sure. I'm not aware of any verb ぐう or ぐる that would conjugate to ぐって. I suspect that this is one of the words that the other character is referring to with their comment, 「語彙が。。。」
UPDATE: See Earthliŋ's post re: the ぐって portion.