Why Do People Migrate? Part 1: Facts

Not knowing that much about Europe, it's hard for me to be sure what all the effects are but I can make a few guesses based on the USA.

As for effect on wages, a lot depends on the existing economy and where the refugees are placed. If the economy is booming and most refugees easily land jobs, they probably won't depress wages but may prevent wages from rising faster - an effect that's speculative and hard to measure. Most countries make it relatively hard for refugees with professional skills such as doctors and lawyers to obtain similar work so high-wage jobs are probably affected less than low-wage jobs. If as sometimes happens the resettlement agencies place refugees where housing is cheap and available, that may indicate a poor job market but refugees may not affect wages much if few of them bother to apply for jobs in such an area.

GDP will almost certainly increase as the value of what the refugees produce is added to the previous economy. At the same time, since at least initially refugees are towards the bottom of the economic ladder this will similarly depress per-capita GDP and to the extent that refugees receive transfer payments it will increase taxes which will mostly be paid by the upper income residents. What with some people fearing for their jobs and others fearing increased taxes, it is easy to see why refugees are unpopular economically even if there were no issues with varying customs.

One last comment has to do with growth. While traditionally growth was considered good and a higher GDP something to strive for, more recently green and other groups have started expressing concern that too many resources are being used and the earth is becoming overpopulated. These people would see additional population as bad regardless of whether they are refugees or native, and want to cut back consumption rather than increase it. This is often seen as a leftish viewpoint as is humanitarianism which believes in accepting refugees, so there is a real conundrum here.