what is more correct?

[bartholomew]

1. You do not even complete half of one circuit at the gym

2. You do not even complete a half of one circuit at the gym

[jno]

both are all right, but 1 sounds a bit better. Better still: You do not complete even half of one ciruit... ('even' should go before the word it emphasises).

[Kinsao]

I'd say, "You do not even complete half a circuit at the gym". I mean, regardless of where I'd put the 'even' (which might be in either place), I think most people would say 'half a' instead of 'half one'. Of course, whichever word you use, you could put an 'of' in front of it... I might say, "You do not even complete half of a circuit at the gym" if I was feeling particularly pedantic (not likely though!)

[Grunty]

I agree with jno.  But as a Pole learning English, you are now considering differences that a very large proportion of English people do not know.  Congratulations - your English must now be very good.