I wanted to find the average Transfer GPA for each of the UCs. So far, I can only find one from UC Davis and UCLA. I was wondering where I can find the GPA averages for Irvine and Santa Barbara.
If there’s an average Transfer GPA Section for CSUs as well, that would be great 
For the UC’s, Transfer GPA by major and campus:
https://www.universityofcalifornia.edu/infocenter/transfers-major
For the CSU’s, you probably have to go to each schools website and see if they post a Transfer profile like SLO.
There is a document called the Common Data Set that all accredited colleges publish. Section D deals with transfer admission. If google - UofX common data set, you’ll find the answer for any school you are interested in.
Of course averages are just that… note that GPA isn’t the only factor for transfer applicants and averages can be impacted by programs like TAG at UCs and by the ‘local preference’ of CSUs. If you aren’t local to the competitive CSUs or can’t use TAG - you should aim to be above the average. If 60% of admissions are getting in with a lower GPA standard, the average simply won’t be competitive unless you are part of that program.The SLO numbers are a great illustration. If you apply to their ag program with a 3.3 and you aren’t local, you won’t make it in.
Use TAG if you can and apply broadly so you have several options.
Good luck.