1. The output of a compiler program is not redirectable (neither as stdout, nor stderr).

Which platform do you use? It works correctly on FreeBSD.

2. Writeln('a','':5,'b'); I think it should write "a b", but it just writes "ab".

Correct.

3. My standard way for "variable size arrays" in BP doesn't work in gpc:

gpc doesn't have such limits, but your example should work.

4. Problem with CONST parameters: PROCEDURE p(CONST c:t)

This is BP, try the EP standard PROCEDURE p(protected c:t) as a workaround.

5. Initialized set variables don't seem to work:

Known bug.

6. Last but not least: Is there anything like TP's Exit (return from current procedure/function)?

A goto with an explicit label. Groetjes, Berend.