After upgrading from Fedora 25 to 26 the default gcc version is now version 7.1.1 and the output of gcc -dumpversion has changed from major.minor.patch to just
gcc -dumpversion
I used both options together:
gcc -dumpfullversion -dumpversion
This seems to work on old and new in a uniform way:
[root@zeta ~]# gcc -dumpfullversion -dumpversion
4.4.7
[jenkins@build-el7 ~]$ gcc -dumpfullversion -dumpversion
7.2.1