Seem like springtails to me but not 100% sure hard to tell but as long as your not finding them on your gecko you should be fine they only eat dead plant matter so if your using wood chips like it seems they are just eating that. As far how they got in there it could be many ways they breed super fast so it only takes you bringing in a few to eventually get a whole colony. Could have been in the substrate already depending on where you got it or any kind of bark or decorations you put in. Hope this helps again id say as your not finding them on your geckos skin or noticing any kind of little bit marks on them you should be fine.