The relevant text of Col 1:23 is:
τοῦ κηρυχθέντος ἐν πάσῃ κτίσει
The verb κηρυχθέντος (keruchthentos) is part of the verb κηρύσσω (kérussó). In Col 1:23 it occurs as a Verb - Aorist Participle Passive - Genitive Neuter Singular.
Note that the tense is "aorist" - impossible to accurately translate into English because English has no aorist tense. In Greek, the aorist tense -
"defines an action that is undefined" (Mounce, Greek Grammar)
The usual (not universal) "custom" to translate the aorist is to use the simple past tense but this does not have to be the case. In Col 1:23, this might thus be translated simply "preached", but this does not properly convey the passive voice because then we would have, "the gospel that preached" which is definitely wrong.
Thus, many versions translate, "the gospel that was preached" to convey the passive voice. However, because the Greek has the aorist tense, and because the preaching of the gospel was not complete in Paul's time nor even to the current time, some prefer to render the sentence in the present continuous sense, "the gospel that is being preached". This is still passive but does not convey the aorist correctly (nothing can!).
Thus, Paul is simply saying the gospel "is preached". If I were translating, the best we could do in English is something like:
... the gospel that you have heard is preached to all creation under heaven