Well, I'm not the best person to recommend for having just used some free samples of GTOWizard or mobile versions of pokersnowie and GTOSensei.
I am in the process of gathering material for studies in addition to starting to familiarize myself with the most used software in the field.
In my little experience with my research I can indicate 3 GTO trainers that caught my attention:
Pokersnowie (in my research it was the most recommended, but I don't remember if it was for the cost benefit, but I remember reading something like its AI being differentiated)
(well, I only receive some lessons from them in my email because I didn't acquire their trainer, but there are many very well detailed and explained hand reviews that caught my attention and the presentation of the learnaware impressed me)
GTOWizard (It was the one I used the most, but the free version allows very few practices per day and it's worth mentioning that it's very intuitive and easy to use... I mean, it's not difficult to use a trainer but I found this one very intuitive)
SimpleGTO, GTOtrainer and GTOsensei are some of the popular ones too.
While no one with more experience comes forward I believe they are good places to start probing.
ps: I don't remember finding any free ones although most of them we can try (trial) or have some daily free 'matches' for training.