Not sure if this will post twice.
To be very clear,
£300000 for two vulnerable adults in nursing or residential homes is not endless funds at all.
Each person has a threshold of £23,250 so two people is £46,500. That leaves £254,500, or £127,250 each before needing to approach LA to ask for public funding.
If self funding (SF) care home fees are for eg £1000 each (I'm assuming a bit more as I'm SE England but also that their pensions and AA (Attendance allowance as can still claim SF, will pay towards, but fees could be less at £600-900 -only OP knows what they are paying) then that really is for eg 127 weeks (approx. 2 1/2 years). Without including spending on the Person who will need personal spending for their needs (which is fine & should be reasonably done) . It's not as long as people think it will cover. (Why do PPs think LAs struggle financially to fund vulnerable people in their communities? Carehomes are expensive as they provide 24/7 care & accomodation. LAs don't have endless public funds available either.)
If it is discovered (& there are various ways) that LPAs gifted away significant sums to others or themselves in this situation, then yes it would be seen as a Deprivation of Assets and that the LPA has not acted lawfully for the Person's best interests as they have taken money needed for their care and support.
It becomes criminal law (theft) not civil law, when a Person lacks capacity OR if they didn't fully understand/can't recall but had capacity.
It's always best to act with caution in any gift giving if an LPA finances in this situation, and to keep it small and reasonable gifts, to recognise that the Person's situation has changed and they need that money for themselves.
Ps. I'm sure OP in asking for advice, will now be cautious and keep.good records.... that her DD and rest of family will understand DGP's situation has dramatically changed and unfortunately can't continue to give them such big gifts now. It's a mindset change.
LadAlive's advice was clear and succinct. I'd re-read his posts if I was OP.