The operative word here is כָּנַס (kanas) which simply means, "to gather, collect". It only occurs 11 times in the OT, 1 Chron 22;2, Neh 12:44, Est 4:16, Ps 33:7, 147:2, Eccl 2:8, 26, 3:5, Isa 28:20, Eze 22;21, 29:28.
In this final occurrence in Eze 39:28, the word simply means, as usual, "to gather". The context suggests that this is a prophecy of the time, then still future, when God will call the Jews back from the lands to which they had been sent into exile. Inasmuch as this would be a gathering for the second time (after the first time at the exodus from Egypt) it could be called a "regathering", but that would be interpretive.
Perhaps there is a sense that before the dispersal to their lands of exile, the Jews were already gathered in their land and after the exile they will return during the regathering; again this is interpretive.
Most versions simply and correctly render the word "gather" (or equivalent), eg, NIV, NLT, ESV, NKJV, NASB, Amplified, ASV, CEV, ERV, ISV, JPS, NET, NHEB, WEB, YLT, etc. Only the CSB, BSB and HCSB have "regather".