I have children attending GESS in the main campus on Jalan Bukit Tinggi, and in general I am happy with the school and the teachers. As your child will be in Kindergarten, he/she would be placed in the Main campus.
GESS is comprised of two sections - German section (German-speaking, German school curriculum); and the European section (English-speaking, International Baccalaureate curriculum). In the Euro section there are many German-speaking children who have very quickly picked-up English as their second language simply through immersion in the language from a young age. In the Euro section German is a compulsory subject, but is offered at two levels -one for native speakers and the other for non-native speakers. The rest of the subjects are taught in English. As your child is still young, picking up another language will be much easier especially at the age of 3. I'm sure you would agree that opportunities for children to be bi-lingual is one of the advantages of living overseas.
As with all schools there are pluses and minuses. A plus for GESS is the school has deliberately kept class sizes down to around 22 to 24 children per class, and there are only 2 or 3 classes per Year group (grade). On the whole, the teachers I have encountered in the European Section have been capable and dedicated teachers. I have no direct experience with teachers from the German section, but I'm sure there would not be much difference in the teaching quality.
The location of the Main Campus on Bukit Tinggi in terms of public transport can be a problem, but the school buses cover just about the whole country. So if you are based in East Coast or River Valley, they can pick up from your doorstep and drop off inside the school compound. In Singapore, school buses carry both a driver and a dedicated child steward (known here as a "bus auntie") who will ensure the safety of the children while travelling and getting on/off the bus. Most parents of kindergarten school-aged children use the bus service. I have friends who live in the East Coast (Bedok area) and their bus trip takes about 30 to 40 minutes during peak hours. River Valley would be a much shorter ride around 20 minutes. Taxis would take about the same time but cost much more, so the bus service is a clear choice.
Alternatives to GESS include the Swiss School, which is next door and also offers a German-speaking curriculum, albeit it is Swiss German. Otherwise, assuming you decide to live on the East Coast you can go for an East Coast located
International School, but these will all be English-speaking curriculum. Examples on East Coast include EtonHouse Primary (Broadrick Road campus),
Canadian International School, United World College (UWC Tampines campus) and One World. All of these have their advantages and disadvantages - but if you want a small school feel then EtonHouse Broadrick Campus and One World are options. EtonHouse Primary is more established as a combined kindergarten/primary school than One World, and has children attending school there from many nationalities including German. Like GESS, it has only about 2 or 3 classes for each Year group. UWC on the other hand is widely regarded to be one of the "top-tier"
International Schools in Singapore, and their comparatively higher tuition fees reflect this. UWC's campus is huge with hundreds of children from Kindergarten all the way up to Grade 12 on the same campus.