At GLA you can get access to the Servisair lounge which is up the stairs opposite Garfunkels. It is landside - you pass through security after leaving, to get to the gates.
Yes, you can go out and in if you wish, but once you are through security you no longer have access.
Is it worth the money? Only you can answer that. You get beer, spirits, cold drinks, crisps, biscuits - there's even a toaster if I remember correctly. I have lounge access (usually not Servisair) for virtually all my flights and wouldn't be without it. Whether I'd be willing to pay £17 or so might be another matter. If it's a wee treat it might be nice, as a one off. Depending where you are flying to you might want to consider buying into the lounge at your return airport. If you have an Egg card you can get a 25% discount via loungepass.com