Diploma

After completing your Diploma, you can go for engineering or try to get a decent job.

Some courses are:
Chartered Accountant (CA)
Masters of Commerce (M.Com)
Masters of Business Administration (MBA in Finance)
Certified Management Accountant (CMA)
Certified Public Accounting (CPA)
Association of Chartered Certified Accountants (ACCA)
Business Accounting and Taxation (BAT)

If you do not have any personal reasons of conditions, you must seek B.Tech (lateral entry) in any subject of your choice to become engineer before taking a job.

After the completion of your Diploma in civil engineering, from my point of view, you should go for the LEET examinations.
LEET stands for the Lateral entry entrance test. This examinations is conducted for getting entrance to candidates directly in to the second year of BE/B.Tech in various colleges who have completed Diploma in any stream.
If you clear this exam with good rank then you will be able pursue you Bachelor's degree in engineering from a reputed college.

You cannot compare them. Diploma gives you an equivalent degree to 12th whereas B.Tech is a graduation degree course.

GRE is not a must for PG Diploma programs, I will suggest you to look at the university requirements for the specific information you are looking for.

You may either join a college through lateral entry scheme offered by your state or go for a job to gain some experience and then decide your specialization.