What's that old phrase? Beauty is in the eye of the beholder.
The trouble is that Gwen needs that extra-special something to explain why Arthur would put his love for her above his duty to his kingdom. (Something very extra-special: Arthur's love survives Gwen's apparent betrayal**.) And I don't just mean beauty. Suppose Gwen was incredibly beautiful (which the actress isn't, though she's not bad looking***); she is still a serving girl****. A prince or king might have such a woman as a mistress, but she would rarely become queen. Marriages in the distant past were often dynastic, driven by the need to consolidate power or avoid a conflict likely to threaten the survival of the dynasty. (The match set up in this episode demonstrated, until it fell apart, this dynamic.)
So Gwen must, at least in Arthur's eyes, offer much more than mere beauty. Yet, as TL Rese pointed out, that spark just isn't there. Or if it is, it's so low powered that the viewer can't see it, despite all the screen time the two characters have shared. Sadly, the love is a given, and has all the narrative truth of the average soap opera relationship, i.e. absolutely none.
** - We know that magic was involved, but it didn't seem to be the only reason for Gwen's actions; and Arthur knows nothing of this magic, so this is not affecting his judgement.
*** - Though she needs to work on her smile, which is often rather too insipid.
**** - If Arthur really was as besotted as he appears, why is Gwen serving anyone? She may not have appeared in public as Arthur's inamorata, but once Uther dies, Arthur would have set her up with a servant of her own. (Publicly, she may have been seen as his mistress, so the truth - that they were destined to marry - could still have been a secret to everyone but the couple themselves.)