i am worried i will be charged extra for buying games because i am in england which uses £ but this site uses $. is there some kind of transaction fee? i use Barclays if that helps
thanks

Not sure, GOG says that all VAT, blah blah blah is included. So you may want to ask your bank about this.

Taken from here:
http://www.gog.com/en/support/website_help/payments_pricing_promos
"6. What are your prices in different countries?
Our prices are always the same, worldwide. It doesn't matter whether you live in the sunny Bahamas (lucky you!) or some place where polar bears roam the streets, it's always $5.99 or $9.99 per game. Our company is based in Europe and VAT/tax is included in the games' prices. This means that GOG.com will always charge you $5.99 or $9.99 per game. However please be advised that we have no control over your bank's international/card transaction fees.
7. What about VAT / sales tax?
All the prices shown on our page are inclusive of value added tax (VAT) and sales tax. So it's always $5.99 or $9.99 per game, no additional costs!"
Hope this helps!

how much would the fee likely be?

"7. What about VAT / sales tax?
All the prices shown on our page are inclusive of value added tax (VAT) and sales tax. So it's always $5.99 or $9.99 per game, no additional costs!"
It says that all taxes are included so you'll pay nothing more than the indicated price on taxes.
As for a transaction fee, referring to Barclay's would probably be a good idea.

At the time of posting
5.95 USD = 4.10752 GBP
9.95 USD = 6.86874 GBP
Plus any bank and currency conversion charges, my own experience is that it's not much, but I don't know about UK banks.

The games i bought on GOG have costed me exactly what i paid for them, 5.99 and 9.99. No extra charges of anykind. I think Europe is the only region where they charge a little more because of that VAT thing.

You avoid these fees by paying with Paypal I think. That's what I use and I've not been over charged from my bank and I live in the UK. Set up a Paypal and use that, its the best way!

IIRC, VAT in the UK is not applied to items under a certain price point. I don't remember exactly what that price point is, but it's certainly somewhere over the £10 mark. So you should not need to worry about VAT.
However, you may want to check with Barclays as to whether they levy an international charge for online transactions handled outside the UK.

A few people have reported additional currency exchange charges levied by their credit card companies, it depends on the credit card and your location. To avoid the whole issue, just follow Romulus' advice: use PayPal.

Even Paypal depends on region. In my case they charge a far higher currency exchange rate than my credit card, so it's cheaper to buy games from GOG with the card than with Paypal. But then in my case, neither adds a specific handling charge.
The only real way for the topic creator to find out, is to either ask his bank or make two test purchases. One using the credit card, the other using Paypal. Obviously, he should buy two games with the same price tag.

I have the same experiences with PayPal.
The few transaction I checked were all from a Euro PayPal account to a USD accounts, mostly PayPals.
In all cases the exchange rate was considerably lower, up to around 3 USD cents less per Euro.

Nope.

I'm with Abbey, I was charged £5.50 for a $5.99 game.
I haven't actually checked how much my $9.99 game was... I'll let you know when the statement arrives in a few days.