Hey, the admission process is based on your 12th and competitive exam jee or gujcet. After merging the marks of these two, your pr is find. U have to fill an acpc form for engineering and in these they provided a rank based on result. If your rank is within cut off of particular branch then you are able to get admission in that branch, otherwise not.

for every branch, the individual rank varies 

for computer engineering, the open general rank varies from 777 to 900

for electrical engineering, the open general rank must be around 2810 to 3000.

for chemical engineering one must have to score around 1738 according to the previous year trend.

for further insight into the cut off details go to the link

http://gtu-info.com/College/LD/LD-Engineering-College-Ahmedabad-LDCE/Last-Year-Closing-Merit-No-Cutoff

all the best !!!!