Vim uses | to chain ex commands. From :h cmdline-lines:
:bar :\bar
'|' can be used to separate commands, so you can give multiple commands in one
line. If you want to use '|' in an argument, precede it with '\'.
These commands see the '|' as their argument, and can therefore not be
followed by another Vim command:
This is followed by a list of commands, which does not include :map or its variants. So, you'll need to use \|. Further down the help is a note about :map, which leads to :h map_bar:
map_bar map-bar
Since the '|' character is used to separate a map command from the next
command, you will have to do something special to include a '|' in {rhs}.
There are three methods:
use works when example
<Bar> '<' is not in 'cpoptions' :map _l :!ls <Bar> more^M
\| 'b' is not in 'cpoptions' :map _l :!ls \| more^M
^V| always, in Vim and Vi :map _l :!ls ^V| more^M
(here ^V stands for CTRL-V; to get one CTRL-V you have to type it twice; you
cannot use the <> notation "<C-V>" here).
All three work when you use the default setting for 'cpoptions'.
So, assuming you haven't modified cpoptions:
map <Leader>c :GitGutterToggle <bar> :set invnumber<CR>
Note that you should use noremap (and the more specific nnoremap, vnoremap, etc.) map commands so that you don't surprised by your mappings.