"is it normal for GAPDH to give a Ct value high around 27? because I read it must varies between 18 and 22."

There is no inherent reason why this has to be true. Some cells may express a lot of GAPDH, others not as much (https://www.proteinatlas.org/ENSG00000111640-GAPDH/tissue). People forget that with qPCR there are some factors that influence Ct values. Mainly, if the primers are OK, the most important determinant (of Ct values) will be the concentration of the target. For example, if I use a column-based method method to extract RNA from millions of cells, I might get more concentrated RNA per uL and start with a higher starting concentration then say a fast cDNA kit that takes 30,000-40,000 cells as input with similar elution values to the RNA column-based method. Hence in this scenario you will see few Ct difference even if you are looking at the same targets, column-based might have say 10,000 molecules whilst the fast cDNA kit method 1000 molecules which is roughly 3 cycles of difference. You know what would happen if you diluted your stock 1:20 cDNA to 1:40 and used it as input for y our next qPCR reaction, it would be 1 cycle lower than the 1:20 dilution.

As per the amplification curve, the curves seem to flatten early to me but my eyes could be deceived. Might be better to see it in log scale if possible with that software. Have you done any sort of dilution testing to see if the primers amplify at R*2 of >90?

Can Kiessling Thank you for your answer and explanation. the kit I'm using for qPCR (TAKARA) requires a cc of template

How does your melt curves look (a gel/capillary electrophoresis screenshot of the PCR products will also be informative)?