If I offer my clients a monthly maintenance service/retainer is that gharar? I mean things could break and I fix it for them or things might not break at all and I het paid for doing nothing.

If the monthly fee pays for defined work, such as scheduled visits and checks, it is allowed, being a known job for a known fee. If it only pays for fixing whatever breaks, so you may be paid for nothing or for a great deal, the scholars count it as gharar like commercial insurance, and it is not allowed.