On second thought, I agree with Richard's basic argument that you should use the article because TG and CG are not proper nouns. This isn't because of any danger that the reader would confuse them with proper nouns, but just because they aren't proper nouns (they're simply abbreviations used for convenience), combined with the fact that they refer to real entities rather than absractions. (If e.g. TG and CG were symbols for mathematical quantities, you could use or omit the article depending on the specific context.)

My argument comes down to this: the abbreviations are only for convenience, and if you used the full expression you would of course use the definite article. You should thus use the definite article with the abbreviation.

As an exception, you can omit the article where terse wording is customary and generally desirable: in table headings, figure captions, and table entries.