Buying with cash is always best. You got the upper hand.
They don't really care about cash these days, since credit is relatively loose. You just have to realize that financing adds a lot of levers to a deal, any of which can be manipulated. It's especially so with leasing.
For sure, cruise this and other forums to get a feel for what people are paying. Check out the websites of the dealers who do volume business below invoice. I won't advertise for any of them here but they are easy to find. Check out TrueCar, Costco, USAA, and any other buying service to which you have access, to see what your tools are.
Get your financing approved by your lending institution in advance. Unless GM is running a promotion, they usually won't match credit union rates.
If you are leasing, you should take time to fully understand how leases work. There are special names for special parts of the lease equation - "money factor," "residual," and "sales tax capitalization" are a few. If you aren't fully aware of what is happening, a dealer can hide $100s in profit in any of several areas. Read a few articles. Read the Edmunds leasing forum where people on the inside will let the residuals slip.
I suggest you download the Leasematic app for your phone. It's very popular over in BMW world where everybody leases. It's a lease calculator that lets you do what-if analysis on every aspect of a lease deal, and it forces you to learn what each of the variables is and does. If you aren't comfortable with that app, you have no business negotiating an actual lease, IMO.
Finally, take all of this information and figure out the deal you want to make, before you go to the dealer. People on internet forums like to talk about how they convinced a dealer to give up all profit in every area of a deal as the dealer bowed down to their immense negotiation skills. But really, unless you buy 10 cars a year, be ready to let the dealer make some money on your deal. The internet will tell you what a good deal on a Yukon is; get that deal and be happy with it.
Tell the dealer what you are willing to do if they do what you want them to do. Then, be prepared to do what you said you were going to do. That's how deals get done.
Then, after you drive off the lot, don't second guess your deal and definitely don't come back here and ask us how you did because everyone will claim they could have done better. Just enjoy the awesome truck.
One other thing - don't trade your car at the dealer. Either sell it yourself or take it to CarMax. Dealers will absolutely rob you blind on your trade, and the information available to value your trade is much less objective.
My advice.