Very high.

Don’t do it. The chances of bricking your laptop are closer to “almost certain” than “maybe.” Soldered RAM is called that for a reason — it’s not a standard socket, it’s BGA (ball grid array) chips literally fused to the motherboard with hundreds of tiny solder balls under the chip. A heat gun is a blunt instrument. You’ll probably cook the RAM, melt nearby plastic, lift pads, or damage the board in ways you can’t see until it’s too late.

Even experienced repair techs use specialized rework stations with controlled temperature profiles and bottom preheaters. A heat gun from Harbor Freight is not that. You might get lucky if the laptop is old, the chip is large, and you have steady hands, but you’re also risking cracking the board or creating cold solder joints that fail weeks later.

This is not where you save a few hundred bucks. If you need more RAM, get a laptop that lets you upgrade it.