Nix31: I'll echo the OP and many others: Galaxy should remain optional
and non-intrusive.
I believe you can increase exposure to your client without displeasing us standalone installers' backup enthusiasts.
First, there is already a big blue
TRY GOG GALAXY in every game page in the Library that is pretty hard to miss.
Second, you can put banners that link to Galaxy in the installers (in my opinion the only acceptable way if you insist on this) and then could keep only one set of standalone offline games installers (up to date, please).
Third, you can also put an ad for Galaxy for when someone buys a game through the website, something like
Thank you for your purchase, click here to download GOG Galaxy and play your games as soon as they're ready. Fourth, when someone receives an email for their purchase (Your order #etc is complete), similar thing as the last one:
Thank you for your order, user. You can review the details of the transaction below. Click here to download GOG Galaxy and play your games with our client. I'm sure you, GOG team, can do all of this and better. There are viable options to advertise Galaxy that don't require making your users going through the hassle of unchecking boxes or closing pop-ups they don't want.
+1
Simple and Efficient. Acceptable to people on all sides of the spectrum.
Those who don't want Galaxy won't install it no matter what.
Those who use it already have it. No need to make them download multiple copies of it.
I use Galaxy, so why would I want an installer to be merged with every game installer?
Most importantly, is it being suggested that someone who can create an account on GOG and BUY a game here and Download the OFFLINE INSTALLER would lack the intelligence to download and install Galaxy on his/her own?
EDIT: The installers show promotional content for other games on GOG. Maybe this space can be used to promote Galaxy if you want.