Gladys is in

>! the [Matsesta Microdistrict](https://en.wikipedia.org/wiki/Matsesta_Microdistrict).

Explanation:

>!<pre>MATS not MAC     1,2 MA
>!PATSY not PACY   3,4 TS
>!SETS not SEC     2,4 ES
>!TSONGA not CONGA 1,6 TA
>!MISC not MISTS   1,2 MI
>!CARS not TSARS   1,3 CR
>!DOC not DOTS     2   O
>!DITSY not DICY   1,2,DIS
>!PITS not PIC     3   T
>!ARC not ARTS     2   R
>!SIC not SITS     2,3 IC
>!ITSY not ICY     2   T</pre>

where

>! each "X not Y" is a word meaning X in which we can replace C with TS or vice versa to get a word meaning Y; we then extract letters at the given positions.

Credit where due:

>! Jay and Omega Krypton solved some of the clues before I did, though as it happens I didn't look at their answers.