I can't offer a documented process for getting the rates setup in GP for MN and CA, but I setup CA sales taxes manually quite a few years ago, so I'm generally familiar with the process.
If you are going to be managing the sales tax setup yourself, then you'll want to be familiar with the details, regardless of whether you get advice or data from someone. Ultimately, you are responsible for your sales taxes and you will have to maintain them.
For CA, I would start by reviewing the CA Board of Equalization web site:
http://www.boe.ca.gov/sutax/pam71.htm
http://www.boe.ca.gov/sutax/faqtaxrate.htm
They provide you with the piles of tax schedules and rates, and you'll probably need to spend several hours reading the various publications, how the tax rates should be applied, and getting familiar with the different layers of taxes (state, county, city, district, etc.). You may also need to get written guidance from the CA BOE to confirm your specific tax requirements, such as Origin vs. Destination tax rules, which are apparently a mix in CA.
http://www.boe.ca.gov/pdf/boe8.pdf
http://blog.taxjar.com/charging-sales-tax-rates/
Once you do get familiar with the requirements and get them in writing, and start to understand the complexities, and see what it is going to take to setup the various schedules, and then begin to realize that you will also have to be aware of any and all changes that occur to those rates--oh, and don't forget the reporting you will have to do in order to remit those taxes--you will probably then understand why most customers use a service such as Avatax.
It could be that your company has very few tax rates to setup, in which case it shouldn't be hard to setup and maintain them in GP--assuming your organization doesn't change or grow. But if your organization will require tax rates for a dozen or more jurisdictions and if your company is growing to new physical locations and states, then you will want to seriously consider a tax rate service vs. DIY.