Gus was a man with a vision. Before he even met Walter he was working to revolutionize meth by using a real chemist to create a far superior product to the usual trailer park hillbilly stuff. The cartel laughed at his vision and killed his first chemist. This strengthened his resolve.
He then met Walter White, a highschool teacher with a magic meth formula who was in over his head. White had no criminal experience and relied on (Jesse) a drug addicted street level cook/dealer to move his product, all the while narrowly avoiding jail and death as they bumbled from one enterprise to another.
Gus offered Walter the deal of a lifetime. He gave him a super lab, he used his pre-existing distribution network to sell Walt's product, he paid him handsomely, and he provided a safe and stable working environment. All he wanted was to replace junkie Jesse with a real chemist (Gale) who could learn the ropes from Walt and eventually run the lab on his own. Knowing that Walter was terminaly ill, and in search of a long term investment, what else could he do?
Jesse apparently gives a hell of a blowjob, because Walter was willing to throw all of that away to keep him as a partner. So, under Walter's direction, Jesse murdered Gale and was able to return to the lab in his place. Gus took this setback in stride, and still in need of a long term investment began grooming Jesse to take over the lab.
Through all of this Gus is managing a criminal empire, and fighting and winning a war against the mexican cartel, while Walt's DEA brother inlaw (Hank) is on a personal vendetta against him. By replacing Walter with Jesse and allowing Walter to live, at Jesse's insistence, Gus was taking a huge risk and allowing the DEA's dream witness to survive. Still he allows Walter to live, but decides he's had enough of Hank. Who can blame him? Hank threatens everything he's worked for.
Instead of just laying low with his family and letting Hank go down, while allowing Jesse to be Gus' well paid chemist, and leaving Gus to run his criminal empire in peace, Walter poisons a child in an elaborate plot to kill the man who made him rich and spared his life by detonating an explosive in a retirement home.
Gus wasn't a good guy, but he wasn't any worse than Walter. For a drug kingpin he was remarkably understanding and professional.
When they introduce the character Mike he's running errands for Saul Goodman, a couple of episodes later he's the #2 man in Gus' criminal empire. What the hell?