There are many schools of thought on how to gain more reviews for your Google My Business listing. Google has a set guideline on what's not against terms and services.
Remind your clients to leave reviews and let them know how easy it is to do. Be sure to reply to reviews as it shows the local area you care about your reviews. This entices people to leave more reviews. Be sure to verify your Google My Business listing. Only verified businesses can respond to reviews.
It is against terms and services to gate reviews. Gating reviews involve redirecting reviews to a piece of software that only posts good reviews and tosses out bad reviews. Buying fake reviews is also against the rules. Any incentivized reviews such as giving out coupons to leave a review are also against the rules.
While it might be tempting to obtain reviews by means against Google terms and service, everyone that sees your profile has the ability to report your reviews to Google. Even more importantly, obtaining fake reviews might be breaking some local laws.