Instead of using \begin{gather} ... \end{gather}, you can use \begin{gather*} ... \end{gather*} to turn off autonumbering for equation. This will not display any number for any equation inside the gather.
If you want to turn off the number for a particular equation inside gather, you can use \nonumber.
Here is an example:
\begin{gather}
a = b+2 \nonumber
d= a+5
\eng{gather}
Equation a=b+2 will not have any number.