I couldn't find any answers on how to use/implement Google Content Experiments with GTM.
I currently have set-up a GTM container with GA code and in the original page for the experiment i have the Google Experiment Code at the beginning of the html tag. The problem I'm having is that not all users are counted in Google Analytics Experiments Report.
- Is it still impossible to use Google Content Experiments in GTM? (How to run Google Experiment within Google Tag Manager - seems a bit old discussion)
- What is the best way of implementing the experiment code in Tag Manager?
It's still impossible to implement Google Experiments with GTM.
However as far as I understand - you implemented the experiment code in the normal way. Did you checked if the experiment is set for 100% of your visitors? If it's less - it's normal to get some visits outside of the experiment and it sounds like this is the case with you.
Other than that - the GTM shouldn't affect the experiment itself in any way, as you don't use GTM for it.