Do not confuse the meaning of physical with our worldly concepts.
Angels, while identifiable with spirit beings, also are associated with having bodies. Some fallen angels have lost their bodies, but still exist spiritually.
Job 2:2 refers to Satan walking to and fro the earth.
It is true that some demons do not have bodies as we know them, but they are still able to engage in actions which result impact physical phenomenon. They can interact physically, throw a man’s body around, engage in a fistocuffs, although not be visible nor touchable by the man. They are in a different domain, which we really do not need to know at present as His Plan for us doesn’t require it.
While many may not believe in the Gen6 basis for Gap theory, IMHO, it is wise not to label it as false, but instead simply accept His Word as it is.
If He had wanted us to know more about it, He has the ability to communicate it, and does if the situation so warrants it. IMHO they are not situations to be coveted nor sought, as that tends to be the domain of witchcraft, which is highly forbidden.
Yes, demons and angels are in their own dimension and that isnt' a dimension we are supposed to try and mess with. Many do and find out the hard way that was a mistake.
And no, I do not believe in the so-called "gap theory" as I find that it does not stand up to what is taught in Genesis nor in looking at the totality of scripture. I believe in a literal interpretation of Genesis for very sound reasons. I believe God's Word for what it clearly says. Six days means six days - no more and no less. If God had meant something else He would have clearly said so. Please no Hebrew word lessons as I have done in depth studies of that as well.
Studied that one for over 20 years also and I find Dr. Morris' and Dr. Whitcomb's analysis to be in line with scriptures their as well....although I came to that conclusion through studying God's Word even before I was aware of those guys.
You are correct that God does not tell us everything (Deut. 29:29a) but there are many other things God does reveal to us (Deut. 29:29b) and the Bible is it's own best commentary; scripture interprets scripture. That is why I look at the totality of the scriptures related to a topic.