Blood tests for oestradiol levels for those using transdermal HRT are often unreliable and is a subject much discussed on this forum, along with discussion and debate about levels required for adequate bone protection. Some older studies have suggested levels as low as 60 pmol/L are sufficient, whereas some menopause specialists recommend levels in the 250-300 region. 175 pmol is pretty low for 4 pumps of Oestrogel though, and if you are still experiencing typical menopausal symptoms, it would suggest that you are not absorbing well.
Do you apply the gel once or twice a day? The gel forms a reservoir under the skin and can get saturated if too much is applied to one area, so applying twice a day may work better, if you've not tried that. Otherwise, it might be worth trying patches, or perhaps Lenzetto spray, which is applied to the forearms, although some women, myself included, find this to be quite weak. But we are all different how we respond to different products. Unfortunately, it sounds like the GP has shut the conversation down by saying your levels are normal, but it should be judged on your symptoms.