The best thing would be to search and learn the geometry basics from some websites. The stuff tested in GMAT is not very difficult but its about application:
1. May be a combination of geometry principles and algebra
2. OR finding a limiting case. For e.g, a rectangle with largest area that can be inscribed in a circle is a square.
I had a question on my test that required application of number properties to find multiple values of a variable (for eg size of a square) and reject one solution since it violated a boundary condition... you get the idea right.
So don't sweat too much on learning complex geometry properties. Focus on the application type questions. OG and GMATPrep have some good ones.
And the fact that you are a Comp. Sc Engineer doesn't necessarily mean you will get great Quant score without any effort. You need to get your hands dirty if you want to work in a field. I have found it the hard way. I am an Electrical Engineer and I always scored 90-100% in all of my math classes, somehow managed to get a 50 in Quant, but I still get stumped by some of the difficult DS and probability/combination/statistics questions posted here.
So put your Ego behiend and solve as may problems as you can. Pay special attention to the GMATPrep ones and more importantly work on your weaknesses.
"To dream anything that you want to dream, that is the beauty of the human mind. To do anything that you want to do, that is the strength of the human will. To trust yourself, to test your limits, that is the courage to succeed."
- Bernard Edmonds