gh-38395: get rid of some sage-eval in gap3-related code
this simplifies the gap3-parsing done in `_gap_return` and then get rid
of ` sage_eval` used on top of that.
### :memo: Checklist
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preview.
URL: https://github.com/sagemath/sage/pull/38395
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
