You don't have a typical business background so schools will be relying a lot on the GMAT to determine if they think you are up for the rigors of their program. Undergrad is a long time ago but the fact that you had some strong math courses will help to some extent. Have you been able to use quant skills in your work? Do you have recommenders who can vouch for those quant skills?
Another option that can help make up for a lower than average quant score is taking some business-related quant courses like finance or statistics. That shows that you recognize it as a weakness & are already taking steps to address it.
It's tough to say how much the GMAT score will affect you at this point. We'd really need more detail to be able to give you strong sense. A lot will depend on the strength of your work experience - in other words, what specific impact you've had, the results you've driven, the business skills that you've used. And your goals are critical as well - they not only form the foundation of your why MBA argument but that are also a way to make yourself stand out. And if you are planning on a post-MBA career that doesn't require much in terms of quant skills then that means they'll put a little less weight on that quant score.
Overall, you do want to try to at least hit school's averages when it comes to the GMAT, but when you are coming from a non-traditional background you ideally want to exceed it.
If you'd like to speak in more detail definitely reach out to Claudia at
[email protected] & she can set you up with a free consultation!