You can see the sponge under the piece in the side shot. It is exerting some pressure to the piece. Exactly how much, I couldn't say with out pulling it off the frame mounting. But if I put pressure on the frame from the back, the piece flexes some at the 1st v groove that is the base line the Arch details come up from before the inner edge lifts off the frame mat.
Also, I'm 99% sure due to the paint colors this outer ring piece is a piece of the Off World Static Gate. It matches a small piece of the inner ring that I have from Legends that is framed/mounted & that cert says its the off world gate.... (The cert for this outer ring piece just says 'piece of the stargate signed by Richard Dean Anderson). As such, it would have been exposed to the outside elements & moved/transported around so it could be bowed & out of wack to what the SGC gate is. This could be why Legends got the segment, it had some damage & the studio replaced it.... damaged segment was sent to Legends and they cut it down to size that didn't have major noticeable damage. So even if I unmounted it, it may not go back to its proper shape.
Heck if I remember right, one of the bonus featurette's said something about the original gate from the Russel movie was left outside for years and was damaged/warped. They pulled molds off it the best they could, cleaned things up, reverse engineered it/repaired it, & made the new gate for SG-1 (& in the process canabalized the mechanicals & updated them for the moving glyph ring). So if the warp is correct it could be due to that.