If the bloods are taken at exactly the same time of day and before you apply the gel that day, and the same place on your body in realtion to where you apply the gel, if the results are from different labs then that could explain the difference as they all have their own methods and ref ranges. Your results should come with the particular lab ref range anyway - although with estradiol it is so variable at different points in the cycle and with different women then it might not mean much.
Also why worry about the actual results? - I mean how do you feel and what are your bleeds like if you have them - that's more important. Is there anything about your HRT that is worrying you symptoms wise? If not then just enjoy life and forget about levels! I'm not sure why the docs are insisting on doing them especially the GP - it isn't necessary unless there is something of particular medical concern or symptoms are worrying you and you can't tell if you are absorbing it or not.
Not sure what the endo is? is this endocrinologist? It's normal for the GP or the menopause clinic ( nurse or gynae) that does the blood tests and monitors HRT - but maybe you have another condition eg thyroid and the endocrinologist is looking at everything? If so - lucky you because I'm not sure this happens that much - although I haven't any experience of how menopause/thyroid etc conditions are monitored together on NHS (or maybe privately?).
That's a bit of a ramble but just to say I wouldn't worry if you're feeling fine!

Hurdity x