Here are some photos I took of the gun in a few holsters. The first photo is the gun in a G-Code XST Holster for a Glock 21.
The gun fits perfect in this G-Code XST Holster made for a Glock 21. At first the gun wouldn't fit in so smoothly but that was due to the holster not being broken in. After a break in period the gun fits perfect and the holster retains the gun and you hear and feel a nice solid click when inserting the pistol.
Here is where we start to run into some problems. These next few pictures are of the gun in holsters made for a Glock 17.
This is a G-Code XST Holster made for a Glock 17/22 which I use regularly for my personal Glock 17 and 22. The WE Tech G17 does not fit into this holster. The picture shows the pistol in the holster as far as it will go before getting stuck in the holster. I guess if this is your only holster you could use it for the WE G17 but it is not recommended.
As you can see the trigger is still viewable and I was able to reach inside and pull the trigger.
Here is the pistol in a Blackhawk Serpa holster made for a Glock 17/22. The gun fits in this holster better than the G-Code XST made for Glock 17/22 but it does not fit all the way. The holster will not lock the gun in place but rather uses a bunch of tension. Again, if this is your only holster then use it but I would recommend picking up a holster for a Glock 21.
Again you can still see the trigger but you cannot reach in and pull the trigger.