nisi jam dudum gaudimonio dissilio, quod te talem video

 jam dudum: Also written as jamdudum or jandudum, now after all this time. [dudum OLD 3]

gaudimonio: As noted by the commentary, the suffix –monium is simply a Vulgar enlargement of the –ium ending. It may have been derived from adjectives that used to end in –mo, -monis.

dissilio: To burst apart. [OLD 2] This verb is more commonly used in poetry (such as by Lucretius, Vergil and Ovid). It has been suggested by R. B. Steele that Petronius may be modelling some of his phraseology after Seneca. In his Dialogues, de Proventia 4.10, he uses the similar words, cruditate dissiliunt (they burst apart from excess food). Seneca wrote condemning such vices as overly excessive banquets and this may be Petronius’ way of satirizing Seneca. Especially given that he has just written in section 39.3, oportet etiam inter cenandum philologiam nosse.

te talem: Niceros is addressing Trimalchio. talis in apposition can refer to the quality/calibre of an individual or that one is an exceptionally good/bad type of character. [OLD 3b] Here possibly referring to the geniality or friendliness of Trimalchio, since in a preceding clause Niceros is delectatus affabilitate amici.

quod: Used as a conjunction.