I assume you evolved your gengar via trading, rather than by obtaining a particularly old haunter who was just about to evolve naturally. (That’s a possibility as well. A little-known one, as neither trainers nor the pokémon involved find waiting that long to be all that pleasant, but it’s still a method nonetheless, which is why you might come across wild steelix or gengar.)
That having been said, even if you receive your gengar back, the act of trading can sometimes shake a pokémon. Certainly, many who agree to the process do just fine afterwards, but there are those who can’t help but feel as if the act of trading trivializes their relationship with their trainers. Some of these cases feel indifferent to their trainers afterwards, and their partners must then work to rebuild their bonds and reassure their pokémon that trading did not affect how much they care.
Other cases are very much like your gengar: afraid that they can lose their trainers very easily. In these cases, it’s important for the trainer to reinforce the idea that their pokémon that they would never give them up and that the fact that they wanted them back means that those pokémon are still wanted. You can do this simply by making gengar an even more important part of your team. Spend more time with him. Give him a little more affection. Work with him through an extra hour of training. Do everything you can to make him feel wanted and loved, and this will allow him to understand that all you wanted was to help him evolve.
So in a sense, it’s normal, but it’s not usual. It’s rare that this happens. (Normally, either of the other two possibilities—in which a pokémon isn’t affected by trading at all and in which a pokémon grows distant—are far more common.) But it’s also reversible through a bit of extra care.
Of course, also don’t rule out the possibility that it could be something else. Communicate with your gengar and see if he just wants to be more affectionate or if he’s actually being protective due to a real and valid threat. It’s most likely simply a side-effect of the process, however.