The url you mention (sitemap.php?forum=2) are not forum urls, these are sitemaps urls. So the rewriting is not delt with the same. For these, you just need to turn url rewriting on in GYM, and then to generate a new .htaccess with the SEO URL .htaccess generator to automatically add the required rewriterules for GYM's urls (sitemaps, feeds and news & map pages).
As you saw, the url to the forum are already handled properly, since GYM automatically detects our SEO url mods. So this had no consequences for the forum SEO, and you can activate GYM url rewriting now safely, as long as you as well activate GYM's zero dupe (but do it after you implemented the new .htaccess and tryed few sitemaps urls to see if everything is ok).
Once done, clear phpBB's cache to update the links displayed in the forum to the sitemaps, feeds and so on.