Can a Shure SM7B plug straight into a PC?
No, and not because of gain. The SM7B has no converter inside it, so there is nothing on its XLR pins a USB port or a pink 3.5 mm jack could read. What the adapter cables really are is explained below.
Short answer: no. Not with an adapter, not with a converter cable that costs eight dollars, not at all.
The SM7B is an XLR-only dynamic microphone with no built-in analogue-to-digital converter. What leaves its three pins is a balanced analogue voltage of roughly −65.8 dBu at speaking level — millivolts. A USB port carries digital data; there is no arrangement of copper that turns one into the other. This is a topology mismatch, not a gain deficit, and the calculator on this page reports it as such: no amount of software or gain solves a missing converter.
What you need is a preamp and a converter, in one box or two. That is precisely what an audio interface is. For the SM7B specifically, look for one that publishes at least 60 dB of gain, or budget for a +25 dB inline booster in front of a smaller one.
The two traps. The passive "XLR female to 3.5 mm" adapters sold as microphone adapters are the worse one. A computer's pink jack is designed for an electret headset capsule and puts a small DC bias on the ring to power it; the SM7B ignores that bias, and its output sits far below what that input expects. You will get a faint, hissy, unbalanced signal and conclude the microphone is broken.
The second trap is subtler: XLR-to-USB cables — the Shure X2u and its relatives — do work, because they are a complete interface hidden in a barrel. But that barrel contains a preamp, and its gain range is the same question this page opened with. Before buying one for an SM7B, find the number.