I've used GB to buy more than 20 handguns. First, you need to fix a value on the item you plan to bid on. My value is bid+shipping+FFL fee which I refer to as "in the safe" value that allows you to figure your max bid.
Once I find an item I want to bid on, I click the "WATCH" button so I'll get a reminder to bid if the auction ends days away.
Bidding requires a bit of strategy. My preference is to wait until 10 minutes before the auction's end before bidding so I can evaluate the other bidders for staying in the game. Some bidders are tire kickers, trolls jacking the price for kicks, resellers looking for a bargain, and serious bidders looking to own the prize.
For example, last week I saw an S&W Model 3913 listed for auction that afternoon. There were only 2 photos and neither were high quality but passable. I could see that the gun was actually a Model 3913NL and this variation usually sells for more than $500. I hit the "watch" button and noted the auction end time. I return to the auction 45 minutes remaining and note that the Big Gun that started the bidding 4 days earlier hasn't answered the latest bid and a frequent reseller has dropped out. The latest bid is below $395 for a model with a fair value of over $500.
I wait until 10 minutes before the auction close giving the new bidder some "second thoughts" time or "OH, damn I might have to actually pay for this gun!" time to consider how to proceed. I fix the "In the safe" price at $500. Shipping is $30 and FFL is $25, so I bid $444.44.
The bidder is in the game and starts to ladder his bids rather than make a MAX BID. The bid to me is $443 so I up my bid to $455 just in case he has set a limit a $450. That takes me $10 over my "In the safe" price but it's still a good value at that price. My added bid to $455 may have caused the bidder to drop or he simply had reached his value limit or psychological limit. I got my gun for $443.
GB sends email messages telling you what you need to do next. If you win, go to the auction page and click on "Proceed to Checkout". There you check to make sure all your billing information name, address, email, and telephone number is correct. Also, select the type of payment. I usually use a USPS money order. Once you've completed this task, click on "Submit the Order to Seller".
Copy down all the Seller's information. Then send a message through the GB message to the seller giving the seller your FFL name, address, phone number, and inform them when to expect your FFL to forward his info via email or fax or with payment. My FFL likes to fax, then email, and lastly hard copy with payment. NOTE: It is best to locate, select, and talk with an FFL near you before bidding.
I've used the GB payment system but mostly call the seller with CC info or go with the USPS money order. I like the USPS Money Order because using the mail to commit a fraud is a no-no and could cause additional agencies to get involved in investigating the crime. Maybe this is true and maybe not, but I find some comfort with this method.
I hope this helps relieve some "first-timer" angst. Good luck!