Euler was a brilliant astronomer, but unfortunately he probably wouldn't be able to get a job in modern astronomy without a college diploma, much less without a high school diploma. He was also an excellent mathematician, though, and I think that is his best bet in the modern world.
It seems likely that Euler would find a job as a programmer (once he got used to the idea of a computer). Programming is one of the most meritocratic fields, and many programming companies don't care about credentials as much as ability. Euler made important contributions during his life to graph theory, topology, analytical number theory, etc, all important in computer science. He was even the one who introduced the concept of a function to mathematics, something which programmers use day-in and day-out.
Likely Euler would end up back in academics eventually. He had a top-notch mathematical mind (see this list of things named after him), and because mathematics and computer science are relatively meritocratic he could publish several papers and use those to leverage himself back into an academic post.
I think Newton might have a hard time in any job he could get without a diploma. He was essentially an academic from the time he was twelve until his death, and apparently disliked manual labor. Living in 2018 without a high school diploma rules out most of the types of jobs Newton would be good at. I think he would do his best to get back into academics: you can get a GED with a simple test, and from there he may be able to find his way into a university to begin his studies anew.
However, I pose an alternate route: religion. Newton was always interested in religion (Christianity, specifically), and he knew Latin well enough to write entire books in the language. In this confusing, modern world, he may turn to the one thing he knew which has changed the least: the church. If Newton, in his new life, became a well known and accepted member of the Christian community, it's even possible that he might find himself on the road to becoming the next pope, although some of his ideas about religion may be a bit too outdated for him to succeed.
Gauss was another stunning mathematical mind (he also has an impressive list of things named after him). It's very likely he would end up in academic mathematics again (although apparently his disliked teaching), but as this route is similar to what I proposed for Euler, here's an alternative: geoscience.
One of Gauss's notable achievements during his life was mapping the Kingdom of Hannover. Today, we have accurate satellite maps of the whole planet, but many of the processes that shape the Earth are still mysterious. It's possible that, uninspired to rejoin the mathematical community, Gauss would embrace some of these new mysteries of our age, applying his precise mind to problems such as how Earth's magnetic field arises and the mechanisms by which water- and wind-borne sand change the shape of the Earth's surface.
Now the real question is: how does Gauss manage to break into this field? Is it possible for him to use publicly-available data to analyze these things and publish answers, or does he need to get his GED and work his way back into academics?
Pasteur has a clear place in the modern world. Biology, especially microbiology, is one of the widest frontiers in modern science. As with all of these luminaries, however, the lack of a high school diploma will hinder Pasteur here. After getting up to speed with modern developments such as genetics (which would take a little while, I'm sure), Pasteur would have the same long academic route ahead of him as I've suggested for these others.
However, there is a movement these days called bio-hacking or DIY Bio which Pasteur could get involved in without any academic credentials. If he can soak up enough of the modern literature about genetics, there are resources available to do experimentation with genetic engineering - even cutting-edge technologies like CRISPR/Cas-9 - at home or in a community lab space.
Maxwell is, of course, known for his extraordinary work on electromagnetism. However, without getting back into academics, there's not much he could do related to this. Perhaps he could become an electrician?
I think it would be more interesting to see Maxwell get into psychology. He apparently had some interest in it while has was alive (like Newton, he was particularly interested in color). This could take two routes: academic (which we're ignoring) or therapeutic. William Garnett, a contemporary who knew Maxwell, once wrote,
Such complete unselfishness and tender consideration as he exhibited for those around him . . . are seldom to be met with.
. . . he never spoke a hasty word, even to his attendants. His self-sacrificing devotion to those he loved was the marvel of his friends.
Combining this human-centered personality with a chance to study the mind up-close and personal could, potentially, be attractive to Maxwell.
Brunel would get into business, no doubt about it. It seems like he would have the savvy needed to work his way up inside a private company. Perhaps more interesting is the idea of seeing Brunel as a technological entrepreneur akin to Musk. Most of his interest seems to have been in transportation infrastructure, so maybe he would take up the challenge of the next big transportation revolution: self-driving cars. Or perhaps, if he could somehow get the money, he would get involved in big infrastructure projects such as the ones going on in places like Dubai and Hyderabad.
How would he get the money though? Maybe he would have a talent for the stock market? Or maybe he would begin again as he did in real life, working as an assistant engineer and making his way up until he ends up in charge.