The WGF and Gripping Beast vikings are pretty much interchangable. People even use parts from one and mix them with the other on the same miniature.
For Celts,, you'll find Warlord's to be noticeably smaller than WGF. Here, WGF is better and cheaper than Warlord.
However be very careful of WGF's Romans. If you are planning on getting Romans at all, WGF's were not designed to rank up properly and you can only build them in certain ways or they'll have weapons arms and shields jutting all over the place and prevent them from being able to be placed in anything like a realistic close order. Also, WGF's Romans don't really line up in size with their Celts, but Warlord's do. Strange, I know.
WGF's Germans are sized like their celts and are fine. The GB Dark Ages Warriors have later round shields and the WGF Ancient German Warband has earlier more Roman looking shields. They'll both work for Late Antiquity, but once you get to the Viking times, the WGF's shields will look out of place if you care about that.
The Norman stuff from Conquest is designed to be compatible with Gripping Beast.
One thing about WGF: You need to really pay attention when posing them as it's easy to end up with unnatural poses. You also may want to trim their necks a bit shorter as the ball socket often doesn't go deep enough into the torso.
All this stuff is larger than
LOTR. Her'es a WGF orc standing next to a
LOTR orc:
GW night goblin,
LOTR Orc, WGF Orc, GWSpace Marine
The reason it looks more like
LOTR stuff and less like WFB/
40K stuff is the proportions are more realistic like
LOTR. I'd mix Tre Manor's stuff and *some*
LOTR with the 28mm historical stuff, but not everything.