How can I make Vim’s `J` and `gq` commands use one space after a period?

:help joinspaces


'joinspaces' 'js'    boolean    (default on)
            global
            {not in Vi}
    Insert two spaces after a '.', '?' and '!' with a join command.
    When 'cpoptions' includes the 'j' flag, only do this after a '.'.
    Otherwise only one space is inserted.
    NOTE: This option is set when 'compatible' is set.

So, you would do a

:set nojoinspaces

to obtain what you desire.

Alternatively, you can toggle the setting with

:set joinspaces!