-(으)ㄹ 테니 is a contraction of -(으)ㄹ 터이니, where 터이니 is the -니 conjugation of 터이다. 터이다 is made of the noun 터 and the special adjective-like particle 이다 (is). 터's original meaning is a piece of land on which something are built, like a foundation, but it is also frequently used in the abstract sense of a basis or grounds for something, like a logical guess or expectation, reason, etc. The reason is often what one intends to do, so it can make phrases in the "I will ..., so ..." form. -(으)ㄹ 터이니 is thus a conjunctive adverbial phrase suitable for making a sub-clause to state a basis or reason for the main clause. It can also be used in the main clause in -(으)ㄹ 터이다/테다/터입니다 and -(으)ㄹ 테요/테야 form, some of which are a little like -ㄹ게요, but with a different formality and register. These phrases are not as commonly heard as the simple and casual -ㄹ게요, so I wouldn't say they are synonyms to -(으)ㄹ게(요). -(을) 거니까 may be used in place of -(으)ㄹ 테니(까), but 테니 usually sounds better in most such cases. e.g. 음식은 내가 다 만들어 놓을 테니까/거니까 너는 아무것도 가져오지 마 = I'll prepare all the food, so don't bring anything yourself. (expressing one's intention/plan) e.g. 내일은 날씨가 굉장히 추울 테니까 웬만하면 집에 있어라 = Stay home tomorrow if you can, as it will be very cold. (grounds for future action independent of one's intention)