Is there a good reason why gJip doesn't turn a paragraph into a line?
If not, how could I make it work that way?
The answer is simple: gJip doesn't work, because gJ is not an operator. See :h operator for which operators exist. The easy workaround is to use visual mode, and then join all the lines, e.g. use vipgJ, because in visual mode gJ will join all those selected lines (see :h v_gJ).
If you want to know the reason, why gJ and also J has not been implemented as an operator, you have to ask Bill Joy (for J in original vi) and Bram Moolenaar (for gJ only available in Vim). My guess is, that using e.g. a motion for the join operation does not always make sense, e.g. the command Jw is non-sense.
answered Oct 27 '19 at 10:05
gJis not an operator, that waits for a motion afterwards. You can however usevipgJto join all lines in the current paragraph. – Christian Brabandt Oct 26 '19 at 13:44vipJworked as well. – Toothrot Oct 26 '19 at 13:46vipJdoes something (slightly) different thanvipgJ– Christian Brabandt Oct 27 '19 at 10:06