You are asking two different questions in the title and the body and they have different answers.
Is it reasonable to have an extremely overpowered gmpc accompany the party early on to let them safely engage with enemies that they can't overcome on their own? No, or at least not generally.
Generally, GMs should not also have a PC. This question has been addressed elsewhere and so I will primarily refer you to the answers to this question: Can the dungeon master have a player character?
However, to very briefly summarize, generally a GMPC will be more likely to annoy the players than improve the game and it will not give the GM the same type of fun as actually playing. Being a GM should be fun, but it is a very different kind of fun than being a player.
There are some occasional exceptions where it can work, but they are exactly that, exceptions. Also, they are best done in moderation. Fighting alongside a character of legend such as Drizzt Do'Urden once can be fun and creates stories that can be told, especially if the GM carefully makes sure that the Legend would not have won without the help of the PCs. Constantly fighting alongside a legend when you aren't a legend yourself makes you a sidekick. Most players do not want to be sidekicks.
If you really, really must have a GMPC, a support character that focuses on supporting the real PCs is likely to be less bad. This question has some answers that discuss how to mitigate some of the issues with a GMPC further, but even there the bottom-line answer is that you generally shouldn't do it: How should I restrain myself when both playing a character and DMing?
Is giving a demigod increased stats reasonable? Of course it can be, but not like that.
It is quite reasonable, even expected, that a demigod would have higher stats than a mortal. I would almost say it would be unreasonable for them not to have somewhat increased stats.
However, your suggested stats of "Str 27, Dex 29, Con 20, Int 21, Wis 22, Cha 24, HP 16, AC 31." Are...incredible. Those are far higher than I would expect a demigod to have -- other than the Hit Points, which are so low for a character like this I wonder if you made a typo (although you didn't mention level or class, so maybe it isn't too low). Still, you are the GM so if you say those are the stats, then those are the stats.
The real problem is that you want to introduce a GMPC with those stats. Other than possibly the occasional cameo, the players should not be outshined by an NPC. As I mentioned and as other questions detail more fully, a GMPC is rarely a good idea at all and, if you do have one, it should not be one that outshines the PCs.