I'm not a fan of GVs, but I admit that closing these bases are far too easy, in fact, in most cases you will find most first line GV fields closed for long periods, runing the fun of GV entusiasts.
On the other hand, the continuous GV spawn feature would make these fields impossible to take if not closed.
What about:
1 - Placing much more fuel depots at these fields, and negating spawn points (except at the base itself) depending on the ammount of fuel at these base. So, if you kill the all the fuel at these bases, the enemy cant send GVs to yours, but still can defend the base itself.
2 - Placing more than one VH, and as hard to kill as those in the air fields.
3 - Add more barracs and much more hard to kill.
4 - If a defending GV is damaged, it cant only "land successfuly" inside an operational VH, undamaged GVs can "land successfuly" at any plance nearby the base.
5 - And spawn the defending GVs randomly at no less than a mile from the VH (no more cheap and safe tiger spawns).