[Garylisk]

Nope. Despite that GXP happens AFTER OVA3, the only advantages to having seen GXP is:

1. You've already been acquainted with Noike.
2. You know EXACTLY what kind of person Seto is, so her meddling comes off as even more comical.
3. You already know that ...

Spoiler:

Katsuhito reveals his true form to Tenchi in OVA 3.

4. You already know that Airi hates being called "obachan" - Plus you already know that she is...

Spoiler:

Tenchi's grandmother.

That's about it, really. Everything else is still new. However, if you were a well-informed Tenchi fan, you'd also already know...

Spoiler:

That Noike came to Earth to be Tenchi's fiancee by orders of Seto. Tennyo was still a big shock for me, though