Since you do have the Marine Engineering background and still you are on the job in the Marine - field but since you are no more interested in continuation of the job in this line, you are interested to clear GATE.
However, there are altogether 22 subjects / streams included with which you can clear the GATE examination and some of the options being Electrical Engineering, Chemical Engineering, Mechanical Engineering, Mathematics, Metallurgical Engineering etc which can be opted to clear the GATE.
You must take a note of this fact that Marine Engineering is not included in the existing GATE scheme and as such you will have to opt for one of the streams included for this test.
Mechanical Engineering is very close to Marine Engineering since some subjects of former stream were taught to you up to 3 rd semester. The next option could be Mathematics since you have had enough exposure in this subject while doing Marine Engineering.
Choice of subjects should not distract your mind while attempting for GATE. This is the only guiding principle.
Once you clear the GATE with the scoring of 99 percentiles and beyond, you will be inducted in the M.Tech programme in the stream for which you appeared.
Your next attempt should be to appear for NET test and clearing this test would pave the way for your Lecturership in the constituent college. While entering into the teaching line in a college, you can take up Ph. D for your better prospects in terms of emoluments and further promotions.