There is ground fighting in Goju Ryu, so it should not be necessary to add Jujutsu for that reason.
I trained in Goju Ryu as well as in Aikido, at different places. One of the teachers at the Aikido school also taught Goju Ryu at another dojo. He once said that every technique that is in Aikido will be found in Goju Ryu, atleast some variation of it. I tend to believe that. I also know people who have done Goju Ryu demonstrations and were asked afterwards what style of Jujutsu that was.
Taught in it's pure form, it is a very comlete and well rounded fighting style, dealing with different ranges, including the ground.
I, like the Goju Ryu and other teachers I have trained with, study different arts for various reasons. They all have something different to offer. Even though Goju RYu will contain everything you will learn at a good Jujutsu school, the difference is mainly in the approach to teaching.
If you like grappling, you will like Goju Ryu. Unless the school either does not spend much time on the grappling, or the teacher never learned it - in which case they will tell you they had to add Jujutsu to make up for what it is lacking.
If you like this school, go to it. But ask a lot of questions - especially about lineage and where the instructor got his training and credentials. You want to be assured you are being taught properly and completely.
If you can not find a Goju Ryu school affiliated with one of the major Goju organizations, you should be able to find curriculum information and compare kata and bunkai, etc. and make sure you are atleast being taught the same things.