Few thoughts.
Best thing about GMPC is that as a GM you can brutally butcher it, to show players how dire the situation is. Though, almost any NPC, who has been with a group for some time can take that role.
Also, every NPC is GMPC. If GM advances NPC (though slower rate than PCs) it can bring great realism (wrong word?) to game world, and make it more dynamic.
In my current SLA ind game I kind of have a GMPC, because I'm the GM 90% of time, and rest of the time someone else GMs, and I want to play. So, I made a character to myself, and he's practically absent NPC always when I'm GMing. Also, he is about half as powerful as other PCs.
I have never liked the idea of active GMPC, ie. a GMs character who is always with group. There is a too large change it will to Mary/Gary Sue scenario. If GM wants to play himself, then rotating GM is in my opinion better solution. But it needs more episodes game.
And back to original question. It is possible, but playing as GMPC is not, in my opinion, nearly as rewarding as playing as a real PC. Players need to think about separating player knowledge and character knowledge. With GMPC GM has to think about separating GM knowledge, GMPC knowledge, player knowledge and NPC knowledge. For some that may come naturally, but for me it is too much work.
If GM really wants to play as same level than other players, then there are actual GMless RPGs out there, which will probably give best results. Inspectres for example is great (as default it may have GM (I don't remember), but at least we have played it many times without GM).