It does seem to me as well that Gmail is extremely strict regarding to the message content when the sender IP/domain is small/unknown. It most probably uses a scoring system as well, so if your server doesn't add much positive score, your message has to look very legitimate for it to go through - it's hard to predict though as the algorithm surely is a lot more complicate (and impossible to study) compared to rspamd or similar. I've heard other major providers are even worse in this regard, though.
