The rule is that there is an evidentiary presumption on a party according to which he will not withhold from the court evidence that is in his favor, including the presentation of testimony, and if he refrains from bringing relevant evidence that is within his reach, without giving a reasonable explanation for this, it can be concluded that if that evidence had been brought - it would have acted against him. This rule applies to both civil and criminal proceedings, and the more significant the evidence, the more extreme the court may draw more extreme conclusions from its failure to present it (see, among many: Civil Appeal 548/78 Anonymous v. Anonymous, IsrSC 35 (1) 736, 760 (1980); Civil Appeal 9656/05 Schwartz v. Ramanof Trading and Construction Equipment Company Ltd., para. 26 [published in Nevo] (July 27, 2008) (hereinafter: the Schwartz case); Civil Appeal 8222/19 Hananya v. Quality Credit Fund, para. 13 [published in Nevo] (December 7, 2020); Civil Appeal 6277/19 Herman v. Weiss, para. 54 [published in Nevo] (March 15, 2022)). In other words, the "refusal" of a litigant to present evidence that in the circumstances of the case is required to be presented, strengthens the evidence that is in front of the defendant, and weakens his version (the Schwartz case, para. 27). and compare to Civil Appeal 4571/22 Habib v. Habib, paragraphs 20-22 [published in Nevo] (July 19, 2022)).
The conclusion, therefore, is that the testimony of the defendant's parents would not have supported his version and would have weakened it, i.e., leading to the conclusion that the apartment was sold to the defendant and not given to him as a gift.
I will add that the defendant's brother was also summoned to testify, since, according to the defendant's version, they are the "victims" of the secret, and according to him, the concealment stemmed from the desire to conceal from them the fact that the parents' apartment was given to him as a gift. The position of the brothers regarding the "secret", when the matter was revealed to them and how, are details that may have significance in relation to the defendant's version.