It depends on which exact route you’re referring to - trains can follow several different routes between Birmingham New Street and Ashchurch for Tewkesbury - including:
- via Smethwick Rolfe Street, Stourbridge Junction and Worcester Shrub Hill
- via University or Camp Hill to Kings Norton and then either via Worcester Shrub Hill or Worcestershire Parkway
A diversion via, and reversal at, Gloucester is possible and some services do this.
In theory it‘s also possible for services to depart from Birmingham Moor Street or Snow Hill and go via Jewellery Quarter, but this never happens in practice.
Assuming you mean from Birmingham New Street to Bristol Temple Meads via University and Worcestershire Parkway (which is the route that most services take), there‘s a succession of tunnels (Holliday Street 307 yards, Canal Tunnel 225 yards, Granville Street 81 yards, Bath Row 210 yards) between the New Street station throat and Five Ways station, followed by the 107 yard Church Road Tunnel between Five Ways and University, the 62 yard Pershore Road Tunnel between Bourneville and Kings Norton, and then nothing more south of there finally Wickwar Tunnel (1,401 yards) between Charfield and Yate.
I would be fairly sure that the highest point is somewhere near University station, which has an elevation of 137m, though someone with exact gradient maps to hand may be able to confirm.