Of course gas is dangerous and should be treated with caution. That being said, the gas valve is a pretty simple swap and is threaded on both connections. Gas rated teflon tape and or teflon dope will be necessary, +1 to soap spraying all connections to ensure there are no leaks. The gas valve operates via electrical signal from the mother board, ours has two connections, reconnect those and use a manometer for setting correct manifold (gas) pressure- that's the complexity of the replacement. You can find a TON of info & videos on the topic. If they sound doable, it is. If it still seems scary, then defer.
For price checking the quote from the HVAC guy (or informing your decision to DIY), you can get the model/serial number of your current gas valve and see what the part only price is.
To the poster who made a big deal about licensed HVAC technicians: 1) this is the DIY discussion forum, 2) You're on a site that regularly advocates having the wherewithal to LEARN THINGS YOURSELF (or did you miss where MMM replaced his own furnace?!), 3) I've met some idiots who are "licensed" in their trade. Licensing seems to be more about rent seeking from the govt than true knowledge assessment these days.