GNU GPL is free for commercial use. The limitations kick in only upon distributing the software.
So, using GPL text editor or image editor or something like that is A-OK. Using a GPL compiler would be OK also, but if you include GPL libraries with the code, you must make the whole thing GPL. Which is why libraries tend to be LGPL instead of GPL. Shouldn't be an issue.
Open source means that the source is open. And the software is usually under a license that allows free use, even free commercial use. Still, it doesn't hurt to check the license.
