Although It's less about guns and more about paying/donating to projects on GPL. If you don't know where to donate, start with Firefox. Every £ matters
Mozilla do spend a lot of money on software development, 220 million last year, out of total expenses of 425 million which came from a taking of 593 million of which 81% comes from Google.
The rest of the money goes into fighting for software freedom, developing infrastructure tools and other things they're very open about.
Personally I don't donate because I prefer to help small open source efforts where a little money makes a big difference, especially protects which I believe could help emerging open source communities grow or inspire more cc content. I'm glad Mozilla exist and that they get so much money from Google and donations
For preserving GPL let me suggest donating to Software Freedom Conservancy which is directly related to fighting abuses of open source / free software / libre software licences.