Definitely need more info in regards to which game mode you are seeing your GL lose to lower geared teams. Also which GLs are you referencing? Some are just bad on defense and can be beat with some non GL teams though there are few left.
You mention GG for instance beating a GL. If the GL and team have low tenacity than the TM gains GG squads get by hitting target locked toons can cause them to run away on taking turns and decimate you.
Also as mentioned the build up of other toons to get a GL gives you solid squads in the process so I can't say the investment is wasted.
If you spent money to speed up the process that's on you since there is plenty of information out there regarding the usefulness and effectiveness of each GL both on the forums/reddit as well as YouTube.
Seems like you don't know the strengths and weaknesses of the GLs you have or how to use them properly. But without more information regarding which GLs specifically you are talking about we can't provide you with a proper response.
Also don't doubt the advantage a GL can give a new player who can beeline for one and achieve it in 6 months without spending any real money