It will become about OP though.
And her DH, & DD's b/f's parents, & anyone closely connected with the couple.
The repercussions of gambling play out through family relationships as more & more strain is put on the primary relationship (DD), rent money is pissed away, cash is stolen, joint accounts drained, right up to forging signatures on loan documentation, or fraudulently borrowing money against property/assets they don't even own.
A couple of PP's have talked about how their relative or partner gambled, but overcame the problem & are now happily cohabiting in LTR's. I am happy that it worked out for them - BUT THIS IS NOT THE NORM. I hope you are able to gently signpost DD to GamAnon so she can read some survivor stories from partners & fully appreciate the severity of mess, anxiety & unhappiness she could be welcoming into her life, & understand what the ramifications are for the future.
At the moment, I imagine she is viewing this 'loss' of the deposit money as a one-off mistake, which she can 'cure' with love.
It is far more likely to be an escalating pattern of behaviour, which will seriously damage her own financial prospects & future security.
It is understandable that she's selling herself the idea that she can fix this mess with love - it's a reaction to what must be near-crippling disappointment. But she's unaware of how bad things could get FOR HER, hence she needs more information.
Hang on in there OP, so that you can be that source of information by directing her to GamAnon. Tell her you will be supportive, but will be following GamAnon advice, & hope that she also engages with it.
I think the best advice you've had is from the PP who said to make it very clear that you love DD & will respect her decisions, but have fears for her future wellbeing if she continues with the relationship. Talk to her about GamAnon & encourage her to explore the site, especially for stories which resemble hers, so that she can see for herself what other young women have been through.