Yes, you're allowed to do that, but there are risks you should heed of:
1. There are many scammers out there, so do your research first to find out a trustworthy swapping fc/person, instead of using the first one that comes by you.
2. If you happen to swap gp with someone who has RWTed gp, it may or may not get you banned too if the said person is caught for the RWT. (Some people claim to have been falsely banned in this manner, but it's hard to tell for sure if that was the case. If both parties have legally obtained gp though, swapping them is 100% allowed.)
3. If you do decide to swap gp, you should minimize the risk: e.g. if you wanted to swap 1b gp, it'd make sense to at least swap it in 2 trades of 500m each (or even in more trades than that, e.g. 4 trades of 250m), as it greatly mitigates the damage if you do get scammed, whilst barely taking longer than one trade.
"These ghastly beasts CAN be slain! They CAN be overcome!" -Borch Three Jackdaws
This is a silly question and I kinda already expect what the answer would be. Are we allowed to trade our RS3 coins to someone to get OSRS coins or is that considered RWT?It is not for me to judge how silly this is but if you will lose a lot in a treacherous deal I know for sure the days of anger & crying will hit you. One way or the other. It really isn't worth doing that as you can play RuneScape your entire life so that's time enough to earn legit your own coins for RuneScape.