There is a ‘Centre glyphs’ script in Glyphs 2 which was pretty useful. Where is it in Glyphs 3?
You do not need a script for that. Better alternate is to set two metrics keys: width
=500(or whatever the width of the glyph is supposed to be) and RSB
=|and then you Glyph > Update Metrics. Downside is that if the sum of LSB and RSB is an uneven number, the metrics key will stay out of sync.
There is still a script: mekkablue scripts > Spacing > Center Glyphs.