Not only Merck, but also Sigma-Aldrich listed glutardialdehyde (25% solution) and glutaraldehyde (25% solution) under the same CAS # (111-30-8). Surprisingly, glutaraldehyde is more expensive (USD 48.70/100 mL) than glutardialdehyde (USD 34.10/100 mL)! :-)

However, the preferred name for the compound (CAS #: 111-30-8) with the formula $\ce{OHC-(CH2)3-CHO}$ is pentanedial (Ref.1). Other names for the same compound included glutaraldehyde, glutardialdehyde, glutaric acid dialdehyde, glutaric aldehyde, glutaric dialdehyde, and 1,5-pentanedial (Wikipedia).

References:

1. Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book); Prepared by Henri A. Favre and Warren H. Powell; The Royal Society of Chemistry: Cambridge, United Kingdom, 2014, p. 907 (ISBN: 978-0-85404-182-4).

Many articles I've read, just mention glutaraldehyde, not glutardialdehyde.

Not only Merck, but also Sigma-Aldrich listed glutardialdehyde (25% solution) and glutaraldehyde (25% solution) under the same CAS # (111-30-8). Surprisingly, glutaraldehyde is more expensive (USD 48.70/100 mL) than glutardialdehyde (USD 34.10/100 mL)! :-)

Does glutardialdehyde and glutaraldehyde are the same compound?

Answer to this question is yes. However, the preferred name for the compound glutardialdehyde or glutaraldehyde (CAS #: 111-30-8) with the formula $\ce{OHC-(CH2)3-CHO}$ is pentanedial (Ref.1). Other names for the same compound included glutaraldehyde, glutardialdehyde, glutaric acid dialdehyde, glutaric aldehyde, glutaric dialdehyde, and 1,5-pentanedial (Wikipedia).

References:

1. Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book); Prepared by Henri A. Favre and Warren H. Powell; The Royal Society of Chemistry: Cambridge, United Kingdom, 2014, p. 907 (ISBN: 978-0-85404-182-4).