I see that this compiler is mostly targeted at "modern standard COBOL", which I guess would be the 2022 standard.
This means - in general - that many COBOL85 code would not be accepted - but it is likely reasonable to either "support enough" to compile and test NIST85 code - or at least do the following:

• get GnuCOBOL 3.2 preview or later
• build it from source
• run make test -j8 COBOL_FLAGS=--save-temps
• try to compile the generated ".i" files with otterkit - those have all copybooks included, all comments removed all line-continuation replaced and the code effectively in free format

Either add "enough" COBOL85 like the comment paragraphs (AUTHOR. and friends) or remove them with sed or by hand.
Ignore others like the ones using ALTER.

Aim for the NC module first, increasing your own testsuite with everything that does not compile and also check for the execution results in your testsuite. Compile and add the features of all program one by one.
This will get you many failures on the first modules - but very reasonable ones, and much less with each follow-up.

And in the end this will get you many of the real world usages of COBOL and some special cases, too.