I didn't see your post initially, but as to why you didn't get any responses, I might hazard a guess. You complain about low frame rate, yet the first thing you say is that you haven't even been able to reach 250fps.
Most people reject the claim that <250fps constitutes a low frame rate. Even your stated low point of 60-80fps is not low by any reasonable measure.
Overall, low GPU usage generally means CPU bottleneck. CPU usage can easily appear low because of how Windows reports it. Cores add up to 100%, so 25% usage on a four-core processor would be 100% usage in a single thread. It gets more complicated with multiple cores and multiple threads.
You do seem to be suffering from a CPU bottleneck, though given that CS:GO uses the Source engine, you should be able to avoid it by turning on multicore rendering. Since you didn't give any details about your settings, that's about as much as I can speculate on.