This means, that gdImageWebpCtx symbol cannot be found in shared libraries used by the module. gdImageWebpCtx is provided by gd/libgd (not php-gd).
readelf -s /usr/lib64/libgd.so.2
nm -D /usr/lib64/libgd.so.2
will list symbols and I'm very sure, that you won't find it there.
I had dozens of similar/compatibility problems with packages provided by Remi Collet (looks like he's very busy to maintain everything et cetera).
Remove all remi packages (
rpm -qa|grep remi), delete remi-release package and install last available php/gg/etc packages from EPEL repository (you may need to downgrade them with
rpm -Uvh package.rpm --oldversion). Then upgrade all packages (!).
If you want to have php 5.5, then I recommend you to download src.rpm's from remi's repo, install src.rpm and recompile with
rpmbuild --target x86_64 -ba /root/rpmbuild/SPECS/php.spec . This way all binaries will be built according to your system.
That's the reason why I never use REMI's php rpms. Mysql ones are good.