If you're planning to be a Moodle as a service provider, then having
free & open access to the source code & access to a large
library of 3rd party plugins, on which you can build & adapt, is an advantage. It means that you can meet
your clients' needs more quickly, cost-effectively, & precisely.
I recommend looking into typical use-case scenarios. For example, I work in English (as a Foreign/Second language) instruction (EFL/ESL). I've recently used both GC & Moodle to produce very similar course materials & found GC both substantially more time-consuming (setting up courses & administering them) & less feature rich/appropriate, e.g. handling multimedia & automated feedback, which is an important aspect of EFL/ESL, & I find formative assessment (AKA 'marking & feedback') substantially easier & less time consuming in Moodle since much of it can be automated &/or organised into efficient workflows. Saving teachers & admins time, who are typically highly skilled & paid workers, can make the difference between financial success & failure for an online & distance education service. For example, I estimate teachers in my field on a well-designed Moodle course can provide corrections & feedback in 15-20% of the time it takes in GC. That's a massive saving in working hours.
IMHO, an over-arching advantage of Moodle over GC, at least in my field, is that Moodle was designed, from the ground up, around a pedagogical model (See: https://docs.moodle.org/310/en/Pedagogy ). This isn't to say that it can't easily accommodate other models, rather that it's designed a set of highly flexible pedagogical tools, i.e. each activity module, e.g. Forums or Glossary, has multiple pedagogical applications & which are explicitly integrated, e.g. Glossary entries can be auto-linked to text anywhere in Forums, Pages, etc.. Whatever specific learning outcome we'd like to achieve, there's usually some way of doing it in Moodle (maybe with some analytical & creative thinking for more unusual use-case scenarios). In contrast, GC was originally designed as Google Docs, for office use - a very different paradigm to education & training & its features are not informed by learning & teaching research, principles or theory (There's around 150 years' worth of knowledge to draw upon which Google don't seem to have taken advantage of - Hubris?).
Another advantage is having tight integration with free & open source multimedia presentation & learning activity tools like https://h5p.org/ (What the elearning industry ubiquitously used to use Flash & SCORM to do). H5P is installed & integrated with core Moodle "out of the box." As well as integrating 3rd party learning content & activities from providers, like Pearson & Cengage (& in EFL/ESL, English Central) with LTI - This alone is invaluable to organisations & institutions that want to get high quality, viable courses & programmes up & running quickly.
On the downside, an LMS, like Moodle, is very different to adapted office & productivity software, like GC. Most people are far more familiar with the office document & folder organisation paradigm & I think that's a major reason why so many novices to online & distance education/training go with GC (but also end up hating it). In contrast, Moodle is a steep learning curve for admins & support staff, teaching/training staff, & students. So a big part of the expense is providing effective IT support & training to them. That's not to say that it's not also necessary with GC but perhaps to a greater degree initially. In my experience, staff & students tend to get to grips with their Moodle courses relatively quickly, especially if 'reminder' & 'how to' information is integrated into them.
Finally, possibly the biggest advantage is the Moodle community. Developers, admins, managers, & teaching & support staff gather here to help & support each other. Because Moodle is so widely used by learning & teaching/training professionals globally, there's a wealth of expertise that users can tap into & find like-minded distance education & training specialists.
This may also be a good place to start if you're new to Moodle: https://learn.moodle.org/
I could go on but I think that you may well have the general idea by now
I hope this helps!